Fair point. I think it would still take a lot of work, though, since Diablo includes a lot of fast-paced, high-powered stuff, while 5e kind of falls apart and turns into a slog at higher levels. To put it another way, it handles up to the heroic level fine, but the epic levels can feel like a drag, and WotC's solution was to mostly publish adventures that stop at level 15. Cutting HP would be a part of it, maybe streamlining some stuff, creating a different inventory system...

So it can be done. But the fact that it's not D&D also means there's a higher floor to how much thought was put into the game, you know? Sometimes designers put the work in, but sometimes they just pick D&D to be lazy or as a cash grab.

Speaking of Adventures in Middle-Earth, I haven't played it, but I heard the 5e edition is actually pretty good. You're right in that Tolkien's fantasy is way different from the high-fantasy superheroics of 5e, but I heard it had great rules for going on a journey, which 5e mostly glosses over (at least in practice).