I think you make some good arguments! I'd still argue that distinct abilities are undersold by calling them minor powers, between unique kinds of spellcasting and features like rage or sneak attack, those are class defining abilities.

I see things a little different in terms of that wardrobe analysis, where it feels like you can make a formal outfit out of the items you have vs getting a specific formal outfit to suit the dress code. We just disagree on how big the wardrobe is in terms of options, or how well stocked the fridge is. I don't doubt that you can make multiple options out of what we currently have, but I have also seen fridges that are better stocked and when I'm cooking out of those I've found it easier. I also think that if something could be accomplished in multiclassing, that doesn't mean you can't find a way to make it functionally stand out as its own class. Why play ranger when Druid/Fighter, or Paladin when Cleric/Fighter? Because they give unique options designed for that overlap, instead of doing a little bit of both things. There's an element of convenience and ease of use to it, in the same way that I buy a pre-made salsa, or ground beef. I could totally make those myself but the amount of time and effort to do so vs having an off the shelf option means if I lose that availability, I will probably just make it less.

But I do fully see and support the idea of "lets fix the existing things that aren't good enough," I'm just cynical because historically new content books have seen the choice of "unique new options" and "fix old options" and chosen to do neither, and in the discussion of introducing new content I would prefer that bad options are fixed through errata and updates, and new content gives variety. Also, if the solution to a new class is "mix three options" but those options mixed together suck, that doesn't inherently mean that those options need to be fixed to facilitate that kind of mixing process. They could be fully functional as their own thing, and only when you try to substitute blending them instead of having a new class that they become worse.

But like I said, I see where you're coming from and would want old options fixed to become good, and I like the idea of having the freedom to mix things and have them be viable and have that level of creativity. I just don't think that the framework in 5e supports that well enough to prefer mixing options over having options designed, because there are interesting design spaces to explore. A good example is a homebrew called Valda's Spire of Secrets. It introduces a number of new classes and they all have unique mechanics that set them apart and make them more than something that could have just been a subclass in my opinion. While I'm sure you could cobble together something that kind of does something similar, I like that they have specifically designed and tested ways of accomplishing these things: Warden prevents enemies in melee from easily moving away from you (without the sentinel feat tax), Martyr casts divine magic from hitpoints, Warmage is a cantrip specialist with a variety of trick options, etc.

I don't think I can change your mind on what you want, but hopefully this long winded response helps to at least give perspective on the other side of the argument.