I do think it's strange that the Unreal fights are temporary, but your "solution" (if I'm understanding right) is to be constantly retuning every endgame fight instead of just one every few months, and possibly rebuild the way the overall instance works to link fights together. That's a huge difference in work.

Everything in the Unreal fights exists in the game and can continue to be experienced, more or less, by playing the original trial synched. Players can already have the self-imposed challenge of trying to beat Coils sequentially without any special remaking. And those need an accessible story difficulty more than they need an Unreal.