That's a bizarre complaint. You don't trust them to not give new psionic spells to the wizard and cleric, but you'd trust them to design and balance a completely separate parallel magic system from scratch. Now I'm not saying that they couldn't fail at both, but the former is objectively way easier to do than the latter.
Because historically a new magic user with their own user system was less likely to be broken than major addition to the current base casting system.Remembering from 3.5E.... Not really? Mind, there's always an argument to make that 3.5E was broken from day 1, but the variant casting systems didn't tend to break anything. The most broken stuff back then was stuff that used the basic casting and just, found ways to break harder
Except Truenaming but, well, Truenaming was broken from day 1, it didn't break the game
Annd 5e already lets spellcasting classes stack spells, power, slots, and features. I fear the psion/wizard or wilder/warlock/sorcerer.