And that would be like saying that teleport is meaningless because you can just walk. If teleport doesn't matter because you can just walk then it's a poorly written campaign. In this matter I 100% agree with Rynjin.
That is putting the cart before the horse.
If the DM wants to run an adventure where the players go to another plane, then the DM should make that possible for any given party.
If the party wants to visit a plane, but doesn't have access to planeshift, then the DM is not *obliged to give them other means.
If the party wants to visit a plane, has access to planeshift but not the right tuning fork, they may **ask to craft or buy one, it's a much smaller ask from the DM than to put a whole archmage or portal.
The DM has many responsibilities, more than any of the players. More than all of the players. The DM can't possibly be asked to make the PCs for the players. The players need to pick up their end of the couch.
TBH if the players have the means to impromptu planeshift then I'd probably end the session early, or take a 1-2 hour break to prepare stuff. It behooves the players to tell the DM what their plan is, so the DM can prep. If the players go off-rails then they should not be upset that the DM needs time to prepare, even if that interrupts the session.
There's four heavy burdens a DM must bear, two are affected by stuff like teleport and planar travel: Adventure prep and world building. Adventure prep takes time, if the players want to craft a tuning fork then that gives the DM time to prep. Then there is world building. The DM may decide that this is a world without friendly living archmages and a world without portals, the players have no right to change the theme of the world. It's a whole lot easier to justify a tuning fork than a portal or an archmage. A world with portals and archmages looks very different than a world without.
There is a huge difference between asking for a portal- ***a world defining element, and asking for a tuning fork.
The same is true for teleport, it's a lot easier to teleport to the other side of the planet, if the players do it without warning then they best get comfortable waiting for the DM to prep.
*the DM may still choose to, but they are not wrong for saying no.
**when they ask to craft a tuning fork they are giving the DM a chance to prep, this lowers the burden to the DM.
***what if the players don't care about that? Why can't we just have fun? You might wonder. Maybe this is what gets the DM excited, gets them motivated to DM. Why are players endlessly catered to and the DM (the one who does all the work) ignored? This deal seems completely lopsided to me.