Yeah, I'm with you on this in a lot of ways. I personally don't mind "multiclass" subclasses... where an entire class's theme is subclassed. Because for the most part, a class's "theme" or "flavor" tends to be rather wide, allowing for dozens if not hundreds of different stories or themes to come out of it. So the Eldritch Knight's layering of the Wizard on top of the Fighter or the Path of the Zealot's layering of the Cleric on top of the Barbarian, or the College of Sword's layering of the Fighter on top of the Bard-- it allows for the overlapping of a very wide thematic idea onto another class but doesn't steal the very specific schtick of the single class with its subclass. The College of Swords Bard/Fighter is still a very different thematic idea than the Fighter Samurai.
But when you have very narrow theming-- basically a subclass's theme-- and you layer THAT onto another class... it just reduces the individuality of the story need for both. The theming of what a Warlock is versus what a Sorcerer is, is already tenuous enough... but to then have both of them gain power from the exact same source just reduces the theming even further.
A PC somehow making a connection to some bizarre Far Realm entity and getting power from it is a really cool idea for a character, but it's an exceedingly narrow character theme-- only one out of tens of thousands of people "in-world" would ever actually have it (except in a specific case of a cult where EVERYBODY would have it.) So to then have two or more different classes give different ways of getting that theme just to me dilutes the flavor. This person made a deal with the entity to gain power.... this person just has the power of the entity running through them.
This person is a monk with the powers of shadow running through them. That person is a sorcerer with the powers of shadow running through them. That person is a ranger with the powers of shadow running through them. And now this person is a cleric with the powers of shadow running through them. YOU get to be shadow! And YOU get to be shadow! EVERYBODY gets to be shadow! To me this just feels like the sum is now less than its parts.
If I want a very isolated character idea of having this bizarre ancestral connection to a dragon somehow and in some way... would it kill me to just play a sorcerer? Do I really need to ask that a draconic druid get made too? Or a cleric domain about a wolf totem? Or a sun soul wizard subclass? When you get to that point I feel like the adage becomes more and more true-- when everything is special, then nothing is.