So I'm coming into this thread fairly late and may be retreading old ground, but I want to return to a point that I think
@Crimson Longinus brought up.
What would the new mechanics for a hypothetical Psion class be? And how would it be distinct from magic such that it would require it's own system?
How is a Psionic telepathically charming a person different from a Bard casting Charm Person?
What's the difference between a Psion shooting a bolt of lightning and a Wizard casting Lightning Bolt?
What's the difference between using telekinesis to pick up something and a Sorcerer using mage hand? Or using Catapult to attack?
Is it just the flavor? To be a bit facetious, we can just make spell slots into "power slots", spells into "powers", and throw in a distinct spell list (power list) with each subclass getting bonus spells (powers) and I think we're basically set.
Because to me, at the end of the day, it's still just magic, no matter how much technobabble you use to flavor it. The end result is basically the same. And since 5e mostly doesn't care about the source of each class's magic or spells, so why make Psion special?