Quote Originally Posted by Person_Man View Post
Put another way, what benefit does the Eldritch Knight Fighter 20 get that a Fighter 2/Full Caster 18 doesn't get?

Extra Attacks? Sorry, but just using a Cantrip and/or spell or spell buffed attacks scales just as well or better for most other classes. And unfortunately the Eldritch Knight can only make 1 attack + Cantrip per round, or 2+ attacks, but never both. A second Action Surge at 17th level is indeed useful. But it can only be used once per Short Rest, and is not as useful as a Sorcerer's Metamagic (which is essentially a spell only Action Surge that can be used multiple times per day) or various Conjured creatures (which multiply your action economy).

Unfortunately, I think that the Fighter just falls behind starting at level 6ish+, and none of its subclasses options can fix it.
An Eldritch Knight Fighter 20 gets 8 attacks per round (with Action Surge) for up to 160 HP of damage, Indomitable, and superb mobility. A Fighter 2/Full Caster 18 gets... 27-ish HP of damage and full spell slots. They're really not the same niche at all. The Eldritch Knight is great at taking down squishy stuff with great Wisdom saves, like enemy archmages. He isn't limited by Concentration. The full caster is generally great at incapacitating one or a handful of closely-clustered enemies and then holding them there until someone gets around to killing them. The quicker the Eldritch Knight can kill them, the better the odds are that they'll stay incapacitated the whole time. These are complementary roles.