Well, if you're trying to balance psionics against magic your first issue is that magic itself is horrifically unbalanced.
There is that, yes, but to be fair, psionics has mostly been pretty amazingly imbalanced, too, so it shouldn't be too hard to balance them against eachother...
Casters always dominate play, and have since I first started gaming back in 2nd edition. I've never seen a campaign that wasn't dominated by the full casters in my lifetime.
I have seen campaigns that weren't dominated by full casters. They were campaigns where no one played a full caster. Technically, that includes all 4e campaigns, since there was no full/half- caster distinction (I guess the closest thing to a 4e half-caster would be a class or build that used both weapon & implement powers, like the Paladin); in 4e, I've even seen a game go well with /no/ casters (all the PCs were Martial classes), at all. Of course, even in 4e (OK, technically it was Essentials), I've seen campaigns dominated, if to a lesser degree than usual, by a Wizard.
Was 4e explicit that psionics come from the Far Realm?
Not exactly, no, it was a little more involved than the UA version. Psionics were painted as a reaction of the World Axis universe to incursions by the Far Realm. Like an 'immune response,' or something. :shrug: There were also psionics who didn't hold with the theory, though there was no denying that aberrations were real, of course, the Far Realm wasn't a plane you could really visit, or even directly confirm the existence of, so, if you wanted to cut it out of your campaign, you could change the fluff of aberrations that they were some ancient foes of the primordials, or that they arise from twisted insane magic, or whatever works...
Someone even mentioned something to that effect in a tweet. Aberrations have different origins. Some are from the Far Realm or influenced by it; some aren't. In fact, 5e has actually been pretty conservative on actually saying "X is from the Far Realm". I think they are leaving a lot of that up to individual interpretation.
5e leaves hard rules crunch up to individual interpretation, opting in/out, and modding, so it would be really out of character to paint fluff into a corner, like they almost did with the Far Realm connection, and have done with the Weave. The Far Realm connection to psionics /should/ appear in the final version of the Mystic, but it should be a side-bar referencing the World Axis cosmology, there should also be some recognition of the prevalence of psionic powers among classic aberrant creatures, like mind flayers and intellect devourers...