toucanbuzz
No rule is inviolate
There was a "reddit" post of some length awhile back about spicing up boss encounters by making the boss have "tiers." Take them down 1/4 (or 1/3, or 1/2, etc) hit points, the boss sheds all "save each round" effects and the excess damage, gains some new powers or triggers some new feature to the battle. I've begun to take this to heart during my conversion of Paizo's Kingmaker (3rd edition).
As others noted, time is key. High level players have access to so many powers that you'll need a suitable reason they can't have a "15 minute" adventuring day, rest up to full, and repeat, so as to challenge them at some point.
On the other hand, it's nice to flex muscles, and that's the challenge of high level play. D&D has done a decent job of allowing large numbers of inferior monsters to be a threat, so it's an art to make that work. Not everything need be a CR15 battle, but every battle should be meaningful at this point.
As others noted, time is key. High level players have access to so many powers that you'll need a suitable reason they can't have a "15 minute" adventuring day, rest up to full, and repeat, so as to challenge them at some point.
On the other hand, it's nice to flex muscles, and that's the challenge of high level play. D&D has done a decent job of allowing large numbers of inferior monsters to be a threat, so it's an art to make that work. Not everything need be a CR15 battle, but every battle should be meaningful at this point.