It seems like people are forgetting something... the Mystic wasn't so huge and unwieldy at the beginning. The very first Mystic came out in 2015 and included just two subclasses... Awakened and Immortal. The second version of the Mystic came out in 2016 and again included just two subclasses... Awakened and Immortal. It wasn't until the third version of the Mystic came out in 2017 that it had all six subclasses.
So what does this tell us? It tells us that first of all... they had no intention of actually releasing psionics three to five years ago (despite when some folks here have claimed), because it was a year between UA releases. And for those three releases, the design they had established didn't really change that much. Between the first two they just bulked the levels from 5 up to 10, and from the second to the third they offered up the four additional subclasses. So at least at the very beginning of the design they thought they were on the right track, they just weren't in any hurry and had not yet decided on a plan of release. Heck... the largest arguments during all that time wasn't the mechanics, but merely the name of the class and that in version 0.2 they expressly said it was the Far Realm that caused psionics, nothing else. THAT'S what freaked most people out back then.
It was probably only after sitting on the six subclass version in 2017 and seeing people's reaction to not only that UA but also the design and reactions to all the other subclass work they did in that time leading up towards Xanathar's Guide for all the other classes that they came to realize that trying to make a fighter-esque subclass and a casting subclass and a rogue-esque subclass all underneath a single class at the expected power level players seemed to require just didn't work. I mean, sure the Cleric has domains that allow for a more melee-bent cleric, and a ranged-bent cleric, and a sneaky cleric, caster cleric etc... but I don't think any of us are putting those domain subclasses on the same level as the full classes they are mimicking. No one is saying the Trickster Cleric is on par with the Rogue... the Trickster is a Cleric with a minor in roguing.
But had they done the same with the Mystic/Psion, people would have freaked out. Heck... we're still getting this now with people rattling off lists of what the psion HAS to have to actually be a psion. So when the realized they couldn't design a Mystic/Psion that had multiple subclasses of all different types AND that players expected those subclasses to be on par with the actual other classes that focused on those things... that's when the whole "the Mystic does too much" idea came about and they switched over to the psionic subclass idea. If people wouldn't accept an Immortal that was Psion first, fighter second... then perhaps they'd more readily accept a Fighter first, psionics second design. Or a Rogue/Monk first, psionics second design. And apparently that has done better.
So the people who are saying that WotC could have Kept It Simple Stupid and completed and released the entire psionics package three years ago... the true answer is no they couldn't. Because by the time the first player-facing book was to come out (Xanathar's Guide in Nov 2017)... they had already gone done their three UA package of the Mystic, built the class up brick-by-brick, and then found out that the single-class with extremely varied subclasses just didn't hold up. It was either overpowered in places, or didn't give enough of what players wanted in other places. Which meant it wasn't ready for Xanathar's Guide, and thus got pushed back (at which point I suspect WotC then hit upon the plan to include the new class within the eventual Dark Sun setting book, the same way they were working the Artificer towards the Eberron book.) But now psionics is stuck until whenever they decide to go back and release a new DS.