Your insistence on "evidence" and "proof" for what is ultimately my opinion smacks a bit of a child repeatedly asking why. Because the last iteration was released like three years ago. Because the last Psionics UA abandoned the concept entirely and instead built upon existing classes. Because.
I mean, in a game that is built around bringing diversely talented characters together in a party to solve problems in the course of adventure, yes being too versatile is a problem. Yes, individual classes can replicate or exceed the Mystic in one sphere. The issue is that the Mystic can perform in all spheres.
A mystic could burn psi points to use abilities at their highest level without being limited by encounter spell slots, like a warlock. They can alternatively spend all of their points on low priced, efficient abilities without having to convert higher level slots at a loss, like a sorcerer. It is hardly balanced when it surpasses the power curve for two comparable base classes in two different ways.
I'll give you that it is different. I won't give you that it is well designed, balanced, or destined for actual publication. Again, it surpasses the power curve of a similar ability on a base class. The class was a sum and total approach to psionics (again, apparently abandoned), so yes it is the fault of the class to introduce mechanics that are poorly supported. I don't fault it for being a UA -- the very purpose of which is to sling stuff at the wall and see what sticks -- I fault it for being so far outside the balancing ethos of the rest of WotCs released material. Hence my original advice to ignore it instead of trying to fix it.
I played a Soul Knife in tiers 1 and 2. I also DMed for a Wu Jen. While fun, I found myself deliberately holding back both in terms of allowing other players to shine and on not selecting the more powerful abilities.
What you are describing is a wealth of circumstantial evidence that borders on preponderance. Perhaps this is because the community at large agrees that the Mystic is broken through both theorycrafting and playtesting (again, the goal of UA). I can't provide you with an aggregate study analyzing every player's experience, but the proof is in the pudding. If the Mystic had any viability, we would have seen a design schedule approximating that of the Artificer (UA, Revision, WGtE, and finally publication), instead we saw a stall out over a longer period (UA, Revision, silence). And that's with fans of psionics tending to outnumber those of artifice/Eberron.
If you like the Mystic, do you. Your game is your game. But for discussion purposes, my view remains unchanged: the Mystic is broken and it offers so little in the way of compelling or balanced design that I find it difficult to believe anyone who says they want to play it for any other reason than said brokenness. Almost any character concept can be built on the better designed and balanced existing classes.