The thing about psionics which I think is what causes questions is the fact that the "spell list" (for lack of a better term) of a would-be psionic class has just been subsumed by arcane casters to the point where there isn't enough obvious differentiation.
If you look at the three traditional main caster classes in D&D (the wizard, the cleric, the druid)... each of them have spell lists that are very different from each other in terms of what they do and what their focus is on. Much of of the druid/ranger lists are nature and exploration related--- ways of dealing with animals, ways of dealing with plants, moving across the landscape, etc. Much of the cleric/paladin lists are related to protection, healing, defending and helping others, and dealing with the undead. Most of the arcane source is about destruction, instantaneous communication and transportation, and deliberate confusion and control.
And that list of arcane abilities? They all overlap with what a psion would be intended to do. I mean we just need to look at all the primary foci of traditional psychics and psionic power and we see that the arcane power source already does all of it.
Psions read minds. So do Wizards. Psions can move things with their minds. So do Wizards. Psions can see into the future. So can Wizards. Psions can control other people's thoughts. So can Wizards. Psions can walk through walls. So can Wizards. Psions can attack other people's minds to cause them physical pain and suffering. So can Bards (Hi Vicious Mockery!)
What else does the psionic power source have for spells/disciplines/whatever that is different than the lists of the arcane, divine, and primal sources and can separate them to warrant the need of its own thing? As far as I personally can tell... very little (if nothing at all.) The Arcane source has beaten psionics up and stolen its stuff. And that's why so many people question the need for psionics in the first place (and most especially the belief that psionics is somehow different than magic.) If you can use the Wizard class and through a carefully curated selection of "traditional psychic ability" spells create someone who does practically everything you'd expect your "psionic character" to do... then all that's different are the fluff ribbons of the class that don't fit. If you then just wipe away the need for spell components and the spell book... what do you have that can't be considered a psionic character?
What is that psionic spell list going to have that does not make it look like an arcane spell list with just the names all different? That's the real issue.