I know the fallacies and frustrations that went into 5e's design. I'd rather not re-litigate them.
And do you think people would accept Bethesda selling TESVI at full price, if it were literally just an expansion for Skyrim, game-breaking warts and all?
And do you think people would accept it if they said that GTA6 was going to be simply a new expansion for GTA5?
Yet major engine updates, radically changing the stuff under the hood, have actually happened, at least for WoW. (I haven't played enough ESO to speak about whether it has had heavy reworks under the hood.) Other MMOs have required heavy reworks of their engine to keep up with the times as well. FFXIV, for example, has already promised some improvements, and has already transformed quite a bit from the version that was limited by the capabilities of the PS3.
You never have a static engine that people simply put out additional content for. Not even in a "live-service" game.
New content for years--but never forever, which is what "evergreen" requires. Modding communities and homebrewers certainly matter, but do you think fans--or modders!--would have been even remotely satisfied if StarCraft 2 released as merely exactly the same game as SC1, just with new animations? (That's a case where it wouldn't have even been possible; Blizzard failed to keep copies of the SC1 source code, and only got a copy returned to them by a loyal fan in 2017, two years after the final installment of SC2 launched.)
I don't mean to crap on these communities. I've been, after a fashion, part of one; I played Master of Orion 2 long, long, long after it should have been put in the bin, because it really was the best option for something like a decade.....
...but that's sort of the magic number, isn't it? After a decade, the world has changed enough that we should expect the tools to change too. You run into limitations: memory, code, features, what-have-you. An MMO that tried to run on completely unmodified code after a full decade would be laughed out of the room--not even Old School Runescape, which is as close as MMOs will ever get to what Pathfinder was, could survive if it tried to doggedly insist on absolutely never ever changing the underlying components.
And I guarantee you that the WoW of today is NOT backwards-compatible with the WoW that launched in 2004. The FFXIV of today is compatible with neither 1.0 (intentionally so) nor 2.0 (natural result of change over time.) The ESO of today is probably not compatible with the ESO of 2014.