It’s not a case of a particular number of classes. It’s the case that more mechanics makes the game objectively worse.
Here is a case in point: the pathfinder kineticist is a pretty exact fit for the “spellless psion” you are so keen on. It has magical abilities that are not spells, and if it uses more powerful abilities its maximum hit points are reduced. So far, so novel. But the problems start to become apparent when you see them in play, and the Pathfinder CRPGs are a good place to see that. Some of the issues:
- It’s powers are so novel that it needs its own set of feats, because most of the standard feats do little to enhance its abilities;
- For the same reason it needs its own magic items because lots of standard items (such as weapons) are useless for it;
- It’s powers have no interaction with other class abilities, making all multi-class options uniformly terrible, and hence the class has very little variety in unconventional build options;
- No one knows what the class’s role in the party is. No one ever says “we need a kineticist!” So they stay on the bench.