A moon druid is arguably OP just by virtue of its subclass features only, and then it gets full spellcasting, up to ninth level, on top of that. Not great design.
Anyone who believes Moon Druids are seriously "overpowered" past about level 8-10 needs to go into remedial math, ASAP. At level 2 they're outright
broken but that fixes down to "grossly OP" at level 3 and then it just degrades from there until they're not significantly OP. But 2-6 or so they are total monsters.
However, re: your general point, which is that because classes are powerful, subclasses don't have a lot of design space, well, that's true, though it didn't have to be true. I don't think your suggestion re: Druids makes much sense. It seems to take the World of Warcraft approach that "Druids are all about shapeshifting". Making a Druids a half-caster would be terrible design, which would ruin the class, because you can't un-make something a half-caster, contrary to your assertion. It would be hideously complicated and mechanically horrible to look upon, and the opposite of 5E's accessible design. You can't fix Moon Druids that way, because D&D doesn't have a design that allows for that. If it was spellpoint based or something, you could do that, but it isn't. Yet. Instead they should have just had the Warden class, which was a half-caster, and "Moon Druid" would have been a subset of Warden, not of Druid (and called "Moon Warden" or whatever).
So I guess my point is to agree with you and not - if you want more design space, you not only have to strip features out of baseline classes, but you also have to create new classes, like if you want a half-caster baselines, you need a half-caster class.
This has always been an issue in D&D, back to 2E at least, note.
You could see it in the Complete Book of [Class]. The less features a class had, the less the Complete Book could replace or change those features. So the early books were kind of pretty sad affairs, mostly stuff that was more down to a few bonus proficiencies or whatever. But then they got to Bard, which was an extremely feature-heavy class, and they could go completely wild. And as such the Bard's Handbook was amazing, with amazing subclasses that are still around now (unlike perhaps any other Complete book), because of the huge design space.
You saw it again in 2E with the later books that let you redesign classes in a points-based way. The more features a class had, the more you could redesign it.
So yeah if classes in 5E had less, we could redesign them a lot more via subclasses. You might even get to the dangerous realm where we're asking why do we even have Clerics, Druids, Wizards, Bards, and Sorcerers, when we could really just have "full caster". At a dead minimum we could cut it down to Divine Caster, Wizard, Spontaneous Arcane Caster.