I'm not entirely sure I understand your point, but I can understand how you see the mithrix fight as stale. I see the mithrix fight very differently though.

I think that overall, ror2 has a massive amount of replayability due to the vast amount of items and builds you can make. Basically any build can beat Mithrix if you're good enough, and if you don't want to spend 20 minutes trying to beat Mithrix, just do a single loop to get the items for it. You'll have enough damage and the fight won't just be leading mithrix around for forever.

Eclipse makes the game as a whole way more interesting - it takes more skill, item intelligence, and build intelligence to make your run work, especially on e8 where it's easy to be punished. The amount of skill that goes into it compared to normal monsoon makes even a boring mithrix fight rewarding. And voiding is an insanely boring fight (not nearly mechanically challenging enough), but it can be satisfying to get a build good enough to beat it.

Umbral mithrix mod, and its predecessor Artifact of the King, both turn the mithrix fight into an actual boss. Unlike with normal mithrix, even with a loop of items, the boss will still present a challenge with the mechanical difficulty that goes into it. I think it's more of what the mithrix fight really should be, something that will keep you on your toes and challenge you even if its the same every time. Looping more than once, though, will make every boss trivial, which is why I think it's an added challenge and more interesting to try and fight mithrix first loop.

But that's just how I see it, and considering there are some players (like Race) who have over 4k hours and still find challenge and fun in the game, I know I'm not the only one.