Sword of Spirit
Legend
The difficulty is in finding bad guys that can reasonably exist in such numbers and at such power levels. As always, heading to the Planes is probably your best bet; a sudden infestation of 80 Frost Giants in your Material Plane setting rather begs the question of why they didn't conquer the whole kingdom back when the PCs were level 1!
Thank you for bring up this point--I was just about to do it myself.
Sometimes I hear stories of the kind of encounters people have at high levels and think, "how is it even fun to play in a world with that much strain to suspension of disbelief?"
To me it most definitely isn't. I barely put up with it in D&D computer games where suddenly the world is infested with super-high powered monsters that exist in sufficient numbers to challenge the PCs, but just make the world seem like it has become absurd.
There actually is a solution in 5e, and you can get it right out of the monster manual--Planar adventures.
Read the roles that the various devils or demons have in their societies. If the MM isn't sufficient, do some online Planescape research.
They literally have armies of creatures with a range of CRs. Instead of dozens of CR 1/2 orcs with some CR 2 leaders and a CR 6 big boss (just throwing in CRs without looking it up), you have dozens upon dozens of "mooks" with CRs of 3-5 led by a good number of leaders of CR 6-9 with some strike forces composed of those leader types, auxiliary behemoths and lieutenants of double digit CRs, and some leaders with CRs near 20. And instead of creating absurd scenarios to get there, you are making an army that actually follows the descriptions given!
Heck, in reality, it's harder to justify running into just one CR 5 fiend out on the planes! So the game does have a built in support for standard D&D combats at high-level, you just have to think outside the Material Plane.