I don't know if the Mystic died on the vine specifically because it was too big and with too many subclasses. Rather, I think it was because what that playtest packet was really about was testing a whole new mechanical system that wasn't spellcasting. So all the "subclasses" were there to exemplify and show the different ways this new system could be used... and had it been incorporated... many of them might very well have become their own classes within this Psionics system, rather than just one Class with a half-dozen subclasses. The same way spellcasting has a whole bunch of different classes that use the Spells system.
But I think what ended up being the issue was that enough people just said in their survey responses that they didn't want a whole new mechanical system on the whole. The system was just too separate from what the game already had but which accomplished the same function. The idea of having a Spells system and a Disciplines system-- both of which had characters doing the same exact things in-game except the players had to use different mechanics to do so... was just not embraced. After all... if a PC is going to get stronger via Psionics with a game result that matches a PC getting stronger via magic... why do we need two separate mechanics systems to accomplish it?
And I think once WotC realized more people didn't want a whole new game system introduced into D&D... the Mystic went away. And in the years since then the argument has always been whether or not its worth making a Psion class that uses Spells, since those that hate that idea REALLY hate that idea. And as a result, it's probably just better to not bother with a Psion class at all. If most people don't care and might not buy/use psionics rules anyway... and the small contingent who would care, and who would absolutely HATE what you'd be selling... better off just not bothering and instead let a 3PP on DMs Guild bite the bullet and produce a Psionics system with one or more Classes for it.