So.... I've been feeling this for a while. While, granted, we got the Artificer in the Ebberon book, there's been a long term dearth of new classes for 5e. Especially in light of the new UA, it really worries me that it appears to be because WotC thinks they can/should they can simply turn every new class concept into a subclass for one of the existing classes.
I think that's a terrible approach, and it really needs to get called out as a problem IMO.
For one, it clutters up the existing classes to the point of making it harder and harder for players to define a path for themselves within a given class once one is chosen. It doesn't help that subclasses aren't multiclass friendly, meaning you can't hybridize concepts between different subclasses for a given class and must choose exactly one out of a perpetually growing list of options.
For two, there are numerous areas where it just doesn't really make thematic sense, either in terms of history/lore or in terms of verisimilitude, such as psionics being an arcane tradition of wizards. I can get on board with Psychic Warrior being a subclass for Fighter. I cannot get on board with the base Psion being a "Wizarding tradition". As it has been in previous additions, psionics should be a fully fleshed out system that can replace traditional magic where appropriate.
For three, it just feels like a lazy way to develop the system. Using the Psionics UA as an example - Psionics really wants to be a complete system that, if needed, can replace magic altogether in a given campaign setting. Setting it up as merely an arcane tradition is absolutely wrong-headed given the history of psionics in D&D.
The bottom line is that if the concept that you're imagining is broad enough that you can easily mentally conjure many different subtypes within that class, then it deserves to exist as a full class and not a subclass. My personal favorite examples of this are the witch and the shaman, which would not be adequately represented by any existing class.