@DEFCON 1 Nah, it's not a "problem with players", it's a problem with the structure of the game. Wizards are supposed to be able to learn all the spells. That's how they're balanced and designed. They obtain spells by learning them from the books of others and so on.
Except wizards DON'T learn all the spells. Yes, they theoretically COULD, but none ever do. Because quite frankly it costs just too much gold to scribe all of them in their spellbooks, no DM ever puts ever single spell out their into their game world for the wizard player to find and then attempt to scribe, and there are just plenty of spells in the Wizard list that are just plain unnecessary for Wizard PCs to take (based on the make-up of the rest of the party.)
So no, I do not in any way believe the wizard class is "balanced" against the other classes in the game because they theoretically could have every single spell in the Player's Handbook available to them. That's not how WotC's "game balance" works.
What matters is the reality of playing the game. And the reality is that Wizard players will select the spells they like and thus usually prepare the same roll-out of spells every morning (with a couple changes here and there based on need.) So if the list of disciplines our theoretical new Psion class would have are the exact same abilities that are already in the Wizard class spell list, then what exactly is the Psion getting that warrants a new class?
Now absolutely... if the class features for a Psion (and the subclass features of whatever subclasses were made) would have game mechanics that were markedly different that what the Wizard, Warlock, or Sorcerer currently have as class features... then yes, I could get behind the idea of an independent Psion class. So for instance, the same way the Sorcerer gets Metamagic (and thus a whole specially subsystem that screws around with the spells the sorcerer casts)... if our Psion had its own subsystem that screws around with its disciplines... then yes, I'm all for it.
But the problem for me is that up to this point, all I've really been hearing for why the Psion HAS to be different is "No spellbook", "no spell components", and "no spell list that includes non-psionic spells". But the first two could easily be made as Variant Class Features of the Wizard, and the last one as I said was merely just a player choosing to play to theme and only select spells for their "psicrystal" that were psionically-themed appropriate.
If someone can list for me all the actual things this Psion class should be able to do that can't already done by the Wizard... I'd be happy to change my tune. But thus far in all the threads we've now had on this... no one has presented ANY psionic abilities that are
different and not already represented in the game.