Again, I agree with all that, but why exactly there needs to be a CAP on accuracy?
Either you didn't read that paragraph or you're ignoring what it says, because I answered that question in the bit you quoted. It's the differentiator.
Normal attacks have to manage risk, because their attacks can miss. Thus you need to have "one more trick" ready when you take your shot because you might miss. That's the risk management.
Normal attacks would have to-hit chance even if there was no cap. That's already a substantial difference. Accuracy need not be hard-capped to be a difference.
Your point seems to be, as far as understand, that without an accuracy cap you can "negate" this difference by having high enough effective skill, and that such "negative/difference" should be irrevocable. Ok, sure, but what is the irrevocable negative of psionics?
Psi has to manage a resource, because the psi pool can empty quite quickly and your "reload" is on a cooldown. So you need to ration your consumption or be ready to manage actionless turns while your pool refills. That's the resource management.
With enough psi cost lowering stuff and enough psi boosters and with fast metabolism you can pretty much never run out of psi points, so this negative can, in fact, be lowered effectively into non-existence.
With minimal planning this "negative" never even comes into play in majority of fights (everything dies before you actually run of of psi).
On top of that, weapons ALSO require ammo you need to actually BUY. Having to constantly spend money to even use your weapon is another (second) irrevocable negative of normal weapons. So what's psionic's second irrevocable negative?
That distinguishes the play types. If your character doesn't have to manage something, then the gameplay loses a lot of complexity. If it were difficult for non-psi to beat the game on Dominating, I'd be less sympathetic to the miss chance but UR is mechanically easy enough that almost any build played well can win; and because it's not based on random maps and spawns like many Roguelikes, the variety has to come from how you approach the problems (encounters) instead of what/where the problems are. Distinguishing the major combat solution types is a big part of why there's so much variety and replay potential. Most of the combat options are for the old-school gamer used to and fond of chance-based outcomes. Psi exists for the player who prefers deterministic outcomes.
The problem is, if a player is OK with reloading saves, 95% cap is pretty much doing nothing.
If the player is NOT ok with reloading, psionics become essentially the only way to reliably finish the game on DOMINATING.
So this "difference" is having either no effect or HUGE effect. And the effect is also random.
Again, I'd be ok with SOME weapons having accuracy cap under SOME circumstances. But sniper rifle missing 5% of the time when shot from stealth on an unsuspecting target is straight up ridiculous.