They established it in 3e, 4e and 5e(with the Mystic). It's clear that WotC considers it to be a full caster. In 2e TSR made it a full caster.
1) There is no Mystic, and "it" established zero. Indeed, they JUST explained it was abandoned in playtest because people didn't like it. Definitely established the opposite of your claim.
2) I just addressed 4e, and you ignored it.
3) Last I checked, TSR isn't WOTC and I was responding to your quote about WOTC.
Nothing is established here, other than a downward trend. We've had one edition (3e) with a supported "established" Psion as a full caster, one half-assed part of an edition (4e PHB 3) with had a Psion as a caster and a bunch of other psi classes that were not full casters which never really got off the ground because it was so late in the edition cycle and which overall saw poor sales (4e PHB 3 I am talking about here - not 4e as a generalization), and a playtest attempt received so poorly it was entirely abandoned (5e).
I mean let's remember here the 4e PHB 3 went over so badly that they flipped the entire edition to something else as an emergency measure to stop the bleeding. I didn't personally hate PHB 3, but it was not going great. This is not a resounding endorsement for that "establishment".
That is not "establishing" anything. That's instead a trend of "we've tried X, and it's gotten progressively poorer reception over time, so maybe it's time we tried something different."
If you don't turn to arguing why a Psion needs to be a full caster, people might just be persuaded to support that trend more. How about you argue WHY it needs to be a full caster, other than "because it's "established" as that?"