Amorim played Bruno and Casemiro because I think they were the best combination when it comes to playing the sort of passes he wanted played for his approach AND defensive "solidity". I am not sure I get your thought here. I thought there was consensus that Mainoo wouldn't replace Bruno in that setup and that his defensive solidity also wasn't at the required level to really replace Casemiro. Now Bruno moved up, something that didn't happen too often in the last year, and there is a spot next to Casemiro - which naturally falls to Mainoo.
I wouldn't argue with that, playing players in their prime certainly is a thing, although, I'd say when thats the objective, nobody at all would have been mad at Ruben, had he played Ugarte (who is the one who is in his prime (physically) the most out of those 4) or played Mainoo since he is about to reach it, not Casemiro who left his prime already. So yeah, standing might also play into it, there is a good chance. But I don't think, there is a very substantial case that this is "a better reason" than the difference in skillset. I mean, when Mainoo played, it often went along with us going down a notch when it comes to attack. He wasn't a worse player then than he is now but he couldn't play the same balls that Bruno and Casemiro dared to play. And I think that is still the case now - doesn't make Mainoo a worse player at all - just that such speculative long balls haven't been as essential in the last few games.
I have a bit of a soft spot for Ugarte but I can see your point here. The drop off in terms of ball playing ability was very very noticable. But I think, Ugarte did quite well in terms of covering space and making tackles when needed. As said, Mainoos ability against the press is way better than every other midfield option we have so I would have tried to play him anyways - but not as a DM where I think, he is still lacking positioning and to a degree intensity. I think, he'd be fine next to another player who is strong in that regard - but if he is supposed to be the defensive one to free up his partner, I don't think thats a good setup at all. But I think, we agree on that.