Diablo 4: Not Meant for Endless Play.

Blizzard has stated that Diablo 4 is not intended to be played forever and will have a pinnacle boss encounter that should prove challenging no matter what your build is. The game isn't designed to scale with your character's level forever, and once you're a high enough level, you'll come up against this boss encounter. The game is obviously designed with replayability in mind, but that doesn't mean you'll be able to do so until the heat death of the universe.