Not wanting two things that are both not great but cover the same ground isn't rational or reasonable? Seriously?
No, there's no reason that it "has" to be those things, but they usually are.
Those are extremely major issues, not trivialities at all. Literally the entire reason to even have psionics in the first place is in those questions. Sure, you can say "because I like psionics, that's why" as your reason to have them both, and that's fine for your game but not for the game.
At the time, I was posting only on reddit, in r/dndnext. My experience there tells me about a third of the people who talked about the Mystic--any of its incarnations--liked it, a third were ambivalent, and a third hated it. And that some of the people who liked it ended up not liking after playing it, and some of the people who didn't like it liked it after playing it (I can't honestly recall anyone saying that they loved it, though).
And I, personally, prefer the archetypes approach. My Psi Knight is quite enjoyable, and this way I don't have to learn another spellcasting method.
Re: rational/reasonable I was referring to the false "has to be", but it's not rational to trash theoretical psionics because magic is bad.
As for "trivial", yeah that is all trivial, and you present no rational argument there, just opinion. It's not stuff that really impacts the game, and the triviality of it is shown by the fact that psionics fit in just fine in previous editions.
As for "Well on reddit..." I was likewise and on r/dndnext at the same time and that's not what I saw, so what, do you want some kind of anecdote battle? Like a rap battle but way more boring? What I did see was that the major objection to the Mystic was a laughable one which was based entirely around it being mildly overpowered (yes, mildly, when you math'd it out) in a 15-minute-workday-type situation. Almost all the "anti" stuff about it that I saw was based on three things:
1) Bad math. Some incredibly bad. I got a ton of upvotes pointing this out, but nowhere near as many as the original bad math dudes. People would rather go with excitingly bad math I guess.
2) The ironclad assumption that nothing could be changed or nerfed. This was the most insane thing there - but I have seen in on dndnext about other material too, it's bizarre (and cuts both ways, they always assume stuff that's good in a UA will stay that way) - it's like they think buffing, nerfing, and tuning don't exist. It would have been extremely easy to fix virtually every issue, in many cases with incredible simple changes. But the anti people acted like that was impossible.
(As an aside, this is very like certain posters on video game forums, every time a unbalanced change goes on a test server, to y'know, be tested, they act like it's the end of the world, and the game will be destroyed imminently.)
3) Assuming D&D wasn't balanced on 6-8 encounters/day. This was constant. I don't think it would fly nowadays when the 6-8 thing is better known and more accepted. It was pretty funny given how many other classes break on a 1-3 encounter workday too.