...you think that Goblins would be an interesting opponent at level 15? I mean, Bounded Accuracy is fun and all, but it has limits. I think a fight where all the party needs to do is let the Cleric cast one spell then walk towards the enemy in order to win would lack a certain something. I think that you're betraying a fundamental misunderstanding of the monster range in 5e. Elementals? 5-7 CR. Giants? 5-11ish CR. Drow? 1/8 - 7ish CR. None of these guys are really going to challenge 15th level characters, except in huge numbers.
Let's talk about the themes that you suggested. For Dragons, you would be needing to either face multiple Ancient Dragons, or you would be fighting a lot of Adult dragons. Sure, you could suggest that 'Giants' would be a good high level campaign theme. But, unless you particularly fancy fighting sixty of the buggers, they just won't be able to do the job. Elemental Evil? Well, uh, Elementals are CR 5 like I say.
I would suggest that in 5e, at least, all of these ideas work perfectly at the mid level, probably by design. Your claim that they would make more sense at high levels just doesn't match how the MM presents these guys. Perhaps that is because D&D high levels just don't match the fantasy source material, really, and belong to a different game entirely, albeit one that we're not given the tools to flesh out.
Did you not read the part where I suggested upgrading the enemies?
Literally the idea is to make the enemies higher CR, adjust any traps or other hazards or effects, and give the enemies magic items/higher level spells. Which is, again, a 3 to 4 page, at most, task per adventure.
like, you know goblins can be CR whtever you want, right?
As for dragons and the rest, yeah. That is absolutely a failing of the presentation of these creatures thus far, and should be fixed. The most powerful dragons, solo, absolutely should be (and at my table are) a challenge for lvl 17 or so PCs, and should challenge lvl 20s with a bit of backup.
Drow, Giants, elementals, and yes, goblins, are certainly capable of not only leveling up, but of finding or crafting powerful items, learning or devising terrifying magics, and summoning things even scarier than themselves.
The MM writeup of a goblin isn't a rule for how goblins must be. It's an expample of the average goblin.
Lvl 15+ parties shouldn't be facing the average example of
anything, and one of the best ways to highlight how "epic" they are is to face them off against enemies that are just as exceptional, or even more exceptional, as they are.
In the meantime, it's pretty easy to level enemies up, and do this yourself, but it would be a great resource to have a book full of leveled up critters, exceptional versions of common enemies, the spells, items and other magics they might use to become more threatening, etc.
also, we are given the tools. Either the DMG or MM has guidelines for modifying and upgrading enemies.