But ultimately they are just hitting and moving, as spellcasters are just casting spells (and moving).
... I don't think you grasp the point I'm making.
The Martials in question move and attack and do so in manners specific to them, with unique mechanics to make them moving and attacking different from each other, whether tactically or in the sheer quantity of their attacks. They are designed, from the ground up, to be good at what they do and to do it in different ways.
Psions, Sorcerers, and Wizards: Don't.
The distinction they get is exclusively in spell availability... and it's not -that- much of a distinction with the amount of heavy overlap.
What makes the Monk and Rogue unique are the abilities they have outside of hitting and moving. There isn't a lot of uniqueness to the Fighter and Barbarian outside of those.
Nah. They are also unique in how they move and attack. Rogues favoring 1 attack per round and either dropping a handful of dice if they hit, or taking the risk for a bonus action off-hand attack if they miss. The Monk throwing off 6 different punches with various riders on half of them and a middle finger for the fun of it as they run away from the rest of the people in the fight...
These things make them distinct from each other even before you get to the noncombat functions. Certainly distinct from Fighters with bonus action disengaging, dashing, or dodging, on pure action economy, alone.
All of that is BEFORE you get into trapfinding or running up walls.
Same thing with Barbarians focusing on a couple of attacks while being a massive slab of meat to tank for the party since enemies have advantage to hit them, always, and they have advantage to hit enemies for crit-fishing purposes and a higher overall chance of landing good solid hits even though they only swing twice. Compared to the fighter swinging four times and often, but not always, relying on heavy armor and a shield for safety... Of course they could also do an Archer build so... even that's got way more variety to it than Sorcerers and Wizards.
And, again, that's before the Barbarian's trapfinding ability. Which, granted, is a lot more "Ow. Found the trap!" than the Rogue's.
Likewise, there isn't much uniquness to the Psion/Sorcerer/Wizard outside of casting spells, but there is uniqueness for the Bard, Cleric, Druid, and Warlock even.
This one I'll give you, though, since it is the crux of the problem I'm describing.
So while my view is broader, it still stands.
Your view is fine and dandy? But it doesn't address the issues I'm raising. Just kinda ignores them while vaguely gesturing towards some kind of argument against them.
It is why I've argued Barbarian as a subclass of Fighter works, and why Sorcerer (and even Psion now) would make good subclasses for Wizard. Frankly, I surprised they focused Psion on Intelligence and not Wisdom. The concepts of Insight and Persception seem more meaningful to a "mentally-powered" arcane caster. Just like Charisma (or even Constitution) makes sense for a "channeling-powered" arcane caster. Leaving the wizard as the Intelligence "book-learning-formula-powered" arcane caster.
Personally... I prefer Intelligence. Insight and Investigation are Intelligence rather than Wisdom skills and better fit the idea of a psychic character who can look into someone else's eyes and understand their motivation or their past...
But that's just me.
And yeah, Sorcerer and Psion as subclasses of Wizard would absolutely work. And I'd hate it. But it'd work.
However, by putting Psion in Intelligence, you are already shifting it to just a "different" Wizard instead of something unique. Obviously it isn't the only issue, just a contributing one since it is there right in your face, so to say.
There's only 3 stats to choose from. Int, Wis, or Cha, it's going to be "A 'different' Wizard" or "A 'different' Sorcerer" or whatever. I don't think that particular aspect really matters much, in the end.
EDIT: Make some of those shifts and for Pete's sake remove 65% of the spell overlap and things would be better in my opinion.
Cutting the 5e spell list down by a MASSIVE margin would absolutely be my preferred way of making room in the Sorc/Wiz classes for more class features and some kind of specific identity in how they do magic... But it's not an option. WotC makes too much money on selling new spells that people go crazy over.
The question begs: what other things could psion, sorcerer, or wizard do which is NOT casting spells to give them uniqueness on par with the other spellcasting classes?
Well!
1) Sorcerers could have distinct casting mechanics from Wizards.
Different spell lists with minimal overlap would be HUGE for creating some light between the two. Giving the Sorcerers a Spell Point system with their 'Sorcery Points' for Metamagic folded into that would go a long, LONG, way to making them feel different. Give them spell point per round expenditure limits, recovery mechanics for some portion of their spell points at the end of short rests, etc.
Maybe give them the ability to specifically burn their Hit Dice to recover Spell Points to represent them using their very body as fuel for their magic. Hell, give them the ability to do that as a bonus action when they're out of spell points but they also take damage equal to the spell points gained.
Since they're all so charming, build in some charm/distraction/social mechanics into the class that allow them to manipulate people in a pseudo-magical way.
2) Wizards could be built around Knowing Things.
Not just the "Scholar" function granting extra skill proficiency. But build up some Wizard-Specific DCs to recognize NPC vulnerabilities due to their various studies. How about a mechanic which allows them to mark a specific target for the rest of the party to gain some bonus to attacking or rolling saves or whatever else exists against that ONE GUY. And it's based on a Relevant Knowledge Check with a DC of 10+Proficiency+Cha mod of the target or something similar. Or 10+Deception Mod or something.
Give 'em some abilities related to finding information on characters, quests, and objects through Research or finding weird Academics, Philosophers, or even Conspiracy Theorists to ask them for information.
3) Psions could be built around Vibes and Dice.
Ever heard of Psychometry? It's Object Reading. Make that a class ability for them to touch some ritual dagger and know exactly who it has been used on. Or talk to the "Ghosts" that hang out around people like some Spiritualist doing seances. Allow them to use Insight instead of Investigation to represent them "Feeling what has happened, here" to add to that identity.
Give them dice they roll as part of their casting mechanic either to try and preserve their power or boost it with direct modifiers that get lost or kept after use, depending on the roll. Or structure their powers as Cantrips and give them spell points they can spend to boost those cantrips in various ways like increasing the damage from 1d8 to 3d8 to make the cantrip deal as much damage as a 1st level spell if they spend the number of points required to do so. You could make -all- their casting "Buffed Cantrips" that way and free up lots of space for other aspects of the character class.
Frankly, there's a TON of really great options, out there, to make them all feel different, even if they're just moving and casting a spell.
