So the sorcerer bloodlines (mind you, they stopped using this term because many of them aren't inherited) have one real crossover (Great Old One and Aberrant) and one squint and you can see it (fathomless and storm are vaguely water and lightning related). You can really stretch and say Divine Soul and Celestial are both healers. And they have made the shadow sorcerer more necromantic to match Undeath. Still, draconic, wild, clockwork and lunar do not match up with archfey, fiend, genie, hexblade or undying in any way. And I could argue the bard, barbarian, ranger and paladin have as many crossover subclass themes as sorcerer and warlock?
Exactly.
If we take the "squint and they're similar
enough" stance, easily half the classes of the game simply disappear in a puff of logic--all while genuinely reducing the number of
well-supported archetypes the game contains.
Barbarian, Ranger, Paladin, and Rogue could all be Fighter subclasses--if you're willing to make them little more than paper-thin mechanics desperately trying to contain a vast thematic package. But you don't see people clamoring for the removal of Barbarians, for example, even though their thing LITERALLY could just be a Fighter subclass feature and would lose almost nothing! (To be clear, I'm not hating on the Barbarian as a class, I think it's good to have and I really quite
like some of the new creative ideas like Path of the World Tree, but those are pretty clearly "okay, what can we
invent that justifies this class?" and not "this is an
obvious extension of the core theme" the way that, say, Cleric domains are.)
Class reductionism nearly always suffers from some degree of special pleading, because any argument which justifies folding existing classes together almost always also justifies merging Wizard and Cleric
or Rogue and Fighter, two things most ultra-reductionist fans refuse to do. It's quite rare to find folks who fully take their own arguments seriously and thus collapse things down all the way to a two- or one-class system. (Two if you decide that "casts spells" and "uses weapons" are enough to justify different classes; one if you don't; both are compatible with most arguments that claim certain classes should be merged with others.)
Of course, a lot of this then actually is rooted in a completely different argument, generally one in the space of "system doesn't matter" and/or hostility to mechanical representation of thematic or conceptual elements beyond the absolute bare-minimum bare-bones elements, which is a common but not universal position taken by old-school fans. Folks who aren't so much into old-school design generally favor having at least
some degree of inherent specialization in class design, so that different class fantasies truly feel distinct, rather than being smushed together into an indistinguishable grey mass.
