Hmm... I'd tend to agree with you because I'm among the few ones around here that really don't undersell Sorcerers...
But I also have to say I rarely got the chance to really play Druid's best spells because I'm usually the one DMing... XD
Like Transport with Plants: seems like a meh spell at first glance, but I could really see, in an long-term "bring that powerful overlord" down campaign, a subplot made by party to make a (Large for any reasonable DM, but the writing allows you, by RAW, to exit through any plant, although you have to enter through at least a large one -and you usually want an escape plan ^^) plant be brought into the chamber of the tyran. Then you can bypass any and every protection barring the ones in the room itself: because the plant is a totally regular plant, there is no Detect Magic until you actually cast the spell. There are obviously high-level countermeasures available, but still...
Wind Walk is also much better than Fly and Dimension Door, even it it has some risks involved: Wizard has Teleportation of course but that requires some set up beforehand.
As for your very long-range tactics...
First, mostly any class is really susceptible to those attacks at first glance, Sorcerer themselves included: only Wizard could maybe quickly react through some Contigency or Glyph of Warding to cast one of the countermeasures you speak of (or other class getting same spell).
Second, it requires YOU as the big blaster to actually accurately see your target at that distance: 300 feet is fair, 600 feet is doable, 1200 feet is very hard.
Especially if Druid decides to change into a Tiny creature: even if a (arguably generous even if it's possibly RAW) DM decided to consider that you still "see" the creature because it's actually in your vision field (aka you "see" as in getting the image somewhere on the retina, but not see as in actively processing part of what your eye gets in your brain)...
It would just take a few feet of movement of said Druid to go behind any natural objet that would provide him full cover, unless he directly wild shapes into a burrowing animal. Good luck hitting him now, Spell Sniper or what, you will be useless. As would be most AOE incidentally unless your DM is really generous (who would defend that a Fireball, because a sphere, would actually burn down deep in the ground up to the full extent of his radius? Not me).
While it's lesser than Greater Invisibility until very high level (because that one does let you act normally, notably casting non-concentration spells), if you just consider the defensive aspect it's Invisibility on steroids: because you are Wild Shaped (which is not a "magic effect" per se), even Dispel Magic nor Counterspell would work. Because it's a non-concentration feature, you can use it after casting any concentration spell you'd like (like, *totally randomly*, one environment spell or conjuration spell). Because it scales with Druid level, you can stay several hours (or indefinitely as capstone).
Another tactic of Druid (honestly much simpler) would be to just cast a Fog Cloud or any of those many lowish-level spells that obscure areas between you and him, preferably close to him, to let him several rounds thinking about the best course of action.
Because you started from that far from him, even with Dimension Door, it will still require two turns to close in: your tactic is actually turning against you...
Third, you need to be the one starting the fight.
If Druid is humanoid form, getting him on surprise should not be too difficult, although you still need to bypass his passive Perception check, which ought to be decent (or high if he took Observant).
If for whatever reason surprise is not an option (confer notably a bit below), then comes Initiative rolls: both have no particular reason to boost DEX, but you have one less ASI: it will boil down on sheer luck, and whether any of you took Alert, or Lucky, or both. And Druid did not have Foresight cast before for any reason. ;)
Nor did he ritual cast Commune With Nature regularly, otherwise your powerful celestial allies will give you out (because he will immediately notice they are not locals).
Nor did he cast Locate Creature (this one is very far-stretched though, unless he also used Scrying before hand and already knew you were coming to him).
Fourth, you'd need quite a bit of luck to down him on the first round.
In fact, you have little chance of killing him on first turn with reasonable builds (so you and Simulacrum taking Quickened Agonizing Eldricht Blast + Agonizing Eldricht Blast for a grand total of potentially up to 2*4*2=16(1d10+5) = average 16*(10) + 8 (yeah, I'm lazy) = 168 damage, provided all hit.
Druid normally built for high-level (so maxed WIS, 16 CON, 14 DEX, medium armor) would have an average of 8+3+19*(5+3)= 20*8+3 = 163.
AC would be at least 16 (Hide + shield), provided Druid neither has a preventive Stoneskin activated when he's in humanoid form (Land has extra slots so it's doable), nor got hands on magical armor and shield that would easily boost his AC to 18-20, nor decided to get a Magic Initiate: Shield because he already learned in the past that some nasty things could come from afar.
So with your Sorcerer, having 5+6=11 to hit, chances to land each ray would be respectively of 80%, 60-70%, 45% (18 AC by Stoneskin + Shield for example), 35% (worst case: AC 20 through magic armor and shield, plus Shield spell activated on first hit).
So respectively 13 (rounding up), 10-11, 7, and 5-6 hits on all your attacks.
With a bit of luck, you have a decent chance of downing the Druid in the best case. For next in line, you'd need several criticals. In all other cases, you failed to down him, and now he has time to prepare either escape or counter-attack depending on context.
The only other big thing I can think of is Meteor Swarm, but even if both of you cast it, it would amount to 2*20d6 (fire) and 2*20d6 (bludgeoning) damage. Provided Druid did not cast Stoneskin as a permanent countermeasure (arguably very low chance: this is a 4th level spell, so much higher cost), he still has Absorb Elements: immediately halving fire damage of both parts, so total of 20d6 on a failed save. So worst case guy will suffer 60d6 damage for a nasty average of 360 damage: yeah, here you can be sure you got him good that time provided cases hereunder are not applicable.
And this is IF Druid didn't cast Foresight (advantage on saves, imposes disadvantage on attacks) when starting his day: high chance if he didn't know he is targeted by a powerful caster or he is not paranoļac like a Wizard ^^, very low chance otherwise. Same with Protection from Energy. ;)
And this is IF Druid was humanoid in the first place: provided you seem to speak about level 18-20 builds here, I don't see any reason for Druid (at least Moon) to not be in animal shape permanently. Which means, on your part, using spells like Scrying (which target Wis, Druid being evidently good at those saves) to try and locate him. Then trying to blow through his THP (which is just NOT doable on a Moon Druid, who has access to Elemental Forms or can just Wild Shape into a 18 AC, 80+ HP as a bonus action as often as he wishes).
Incidentally, there is no reason either why such a high level Druid would not have basically every living animal being in his general trending area aware of his existence and either respectuous or fearful of him, so feeding him information one way or another. So you would also need to approach covertly if you want to take a chance at your surprise attack.
And apart from having probably several creatures ready to help/defend him without order (although I admit that is 100% DM dependent ;)), Druid can himself invoke powerful Elementals or Feys to help him. I have no useful opinion on comparative worth of high level cast of those compared to Planar Ally (especially since I don't know well the creatures you speak of) but I'd guess the superior action economy of a group (especially when some creatures *cough* Pixies *cough* can cast spells) could counterbalance the superior stats and ability of a single creature (or two, *maybe* three). Technically there are no limits apparently to the number of creatures you could bind with the Planar combo, but I'd expect any DM to restrict one way or another (especially since you actually "invite" creatures to your plane to bind them forcefully for a great amount of time, to send them on a very dangerous mission: the godly entity you are calling to won't appreciate the fact you are using its creatures as scapegoats).
All these creatures would also need tremorsense or great tracking abilities to still find the Druid escaping underground... If he didn't directly use Transport by Plants to quickly escape to a safe haven from where he can ramp up his personal protection against your next attempt.
Sooo... I really don't think it would be THAT easy in a properly made world, for a Sorcerer/Warlock to kill Druid: it would be certainly be doable but with a pretty great amount of preparation and without being totally devoid of risks. ;)