It would be ideal if psionics continued it's trend of being mechanically distinct from spellcasting, but 5e wasn't built around the premise of the traditional psionic character. Despite being an optional system, great pains were taken to make multiclassing viable and allowing spell using classes to hybridize with little fuss. WotC has shown they are perfectly happy to create many new spells and subclasses, but are obviously not as devoted to developing new classes.
So the best we can probably hope for will likely be something like Pathfinder's Occult classes or something. I'd be happy to be wrong on this point, but it is what it is, we may be forced to wait for some third party developer, unless a fan-made psionics system that is widely adopted appears.
Having said that, I don't think 5e's design space is so narrow that it couldn't have an alternative to a spellcaster who produces supernatural effects- I think way upthread I mentioned how the Rune Knight does this, and could be expanded into a "Runecaster" class that produces magical effects that aren't specifically spells.
But WotC is probably of the mindset that the juice has to be worth the squeeze, and will likely take the path of least resistance.
The game will, however, get more complex over time as they attempt to support more campaign settings. We've already seen that they are willing to fiddle with how elements like Backgrounds and Feats function in the Dragonlance UA, they have experimented with more class-agnostic subclasses, and the Mystic, while terribly balanced, at least shows that someone wants to open up the design space of the game.
Player reaction, so far, has been mixed at best. A lot of people have strong opinions about what 5e should be, and should not be. Psionics has traditionally gotten a bad rap in the past, for some fair reasons, and some unfair reasons. This is no doubt why we have yet to see a Tome of Magic or Tome of Battle style expansion to the game rules, and no doubt never will- new content will be attached to new settings for the game.
Dark Sun isn't the only psionic setting, of course, but it's the one with the strongest link to psionic powers, followed by Eberron (due to the Quori and the Kalashtar). If they are working on Dragonlance next, they won't be too worried about psionics.
I don't really know how anyone can make their voices heard that there are people who strongly want psionics and want it to be mechanically and thematically distinct from arcane magic- I don't know if the community of players who want these things is large enough to make a real impact.
But I will try to answer the question posed by the thread. What's the big deal about psionics?
It's a nebulous and divisive concept that some care for, and some do not care for. It hearkens back to the very early years of the game, and it's roots in science fantasy that have been largely rejected in favor of "Ren faire with magic" campaigns.
It's easier to describe what psionics fans don't want, than what they do want. And it's easier for those who aren't psionics fans to tear it down as being "historically unbalanced and not a good fit for my Tolkien-derived fantasy game with the Dungeons and the Dragons".
We won't get a good psionics system as long as the game developers don't have a particular passion for it. Or the player base at large. All we can do is try to educate people on the fence, and hope for the future.