If you're willing to reflavor character options, entire classes can be eliminated. The game could be boiled down to a purely effect-based system, with no need for future rules supplements. There are games that work that way, and people really like them. (I'm a huge M&M fan, and it follows this strategy.) But D&D is not like that.
Flavor is baked into the character options in D&D. This can be extremely helpful at open tables and Adventurers' League games and for new or casual players. But it leads to two difficulties:
1) If the character concept you want doesn't exist yet, you need to resort to house-rules or reflavoring. Some people are more comfortable with this than others. Like, if one of my players wanted to be Elsa, it's pretty easy to refluff sorcerer's draconic bloodline (silver) into something more elemental-oriented. But a lot of people would find that unsatisfying. (We can argue whether they are right or wrong, but we won't change their minds. They're not reading this forum.)
2) If character options are redundant, it makes the game more confusing, and is a waste of space. This is my problem with some of the UA options. "Knight" is already a background option (it is a noble variant). Right now, you can play a classic "knight" archetype by taking that background and selecting fighter class and riding a horse and wearing shining armor. You don't need a Knight subclass. And if they introduce a Knight subclass, what does that say about all the "knight" characters who don't take it? Are they less knightly by comparison? The game just doesn't need any of that.
Some of the UA subclasses are better than other in this regard. Like, we could debate for a while whether Samurai should be a background, a martial archetype, or just another word for "noble" in a Japan-esque setting.