Diablo 4 isn't "intended to be played forever" and there's a "pinnacle boss encounter" at the end

gamesradar.com:

Diablo 4's Nightmare dungeons will scale beyond level 100, but Blizzard says there's something of a final boss encounter that's designed to be your character's capstone.

In a group interview GamesRadar+ attended, associate game director Joe Piepiora confirmed that Nightmare dungeons will keep getting more challenging even past level 100, but there is a limit to the game's difficulty. Piepiora said Diablo 4 wasn't designed to endlessly scale with your character and that you'll eventually face a "pinnacle boss encounter" that'll test the limits of even the best level 100 builds.

"[Diablo 4] is not intended to be played forever. So there are creatures that you will continue to fight at higher and higher difficulties [beyond level 100], but this is content where you'll be kind of pushing yourself to see how far you can take your build, rather than trying to reach some endless grind of rewards as time goes on beyond level 100."

Piepiora adds that there will be an unspecified point past level 100 where you've "received the maximum power of gear," and at that point you can reroll and respec your character until you feel you've maximized your build. And then, and only then, according to Blizzard, will you want to take on the defacto final boss of the Nightmare dungeons. An encounter which Piepiora suggests will take you several tries before having a chance at beating.

"At level 100, we do have a pinnacle boss encounter we want players to engage with that's been balanced so that it's extraordinarily, extraordinarily challenging," he said. "Players that reach level 100 are going to have an extremely difficult time on this boss encounter. And the expectation is that you take your class, you understand your build, you've maximized