MoonSong
Rules-lawyering drama queen but not a munchkin
Sure we have already a lot of mechanics for representing character concepts: classes, subclasses, backgrounds, feats. But this doesn't mean it has to end with these. Each of the existing mechanical option comes with limitations:
- base classes always start from level 1, so you can't use them to represent something that is already meant to start at high level
- subclasses are tied to a single class, so you can't use them to represent training that is available to anyone or maybe not everyone but at least across many classes
- backgrounds are not acquired later in the game and are anyway very limited in what they can grant you, so you can't use them either for "prestigious" or unique concepts
- feats are small and don't represent a progression, so you can't use them to represent concepts that don't fit the value of a feat (unless you introduce feats chains) and even there it is difficult to fit larger mechanics (e.g. spellcasting) in a feat chain
This is not to say that we need prestige classes. If you look back at 3e prestige classes, you can find that at least 80% of them can be converted very well into subclasses or feats or feats chains (I have done a preliminary study myself on converting Rokugan prestige classes).
But I would not close the door to prestige classes in 5e. They could be just another tool in your toolbox, which nobody is ever forced to use.
Regarding updating previous UA articles, it would be really interesting to know what is WotC's plan...
We already know that some UA material is making its way into the upcomig Adventurer's Guide. So it's very possible that really they are using UA as a playtest ground (i.e. "beta versions"), and then they collect feedback, update stuff, and sell the final version with a book. Only if the beta version proved too shaky and the update needed was major they will probably show the update on UA again. And in this case, it might depend on whether they have a plan for publishing it in a book any soon, otherwise it'll take time before seeing version 2 because probably a lot of people would rather see something totally new in UA.
As for your two examples, my guess is that Psionics are such a huge thing that we will likely see version 2 and maybe version 3 in UA before they publish the final version in a book, while Eberron material was probably near-ready and we'll see the final update only in the book itself.
But the atificer and the warforged weren't that well received, the first one would have to be rebuilt from scratch even...