what you say is true, but you forget to understand the reason for this
the Italian championship is very tiring, think Inter lost to Sassuolo twice, a team that went to Serie B
Every game in the Italian championship is difficult and important, you can`t save resources and effort, it could be damaging and counterproductive
so you understand well that it becomes difficult to do demanding things like UCL
I think every league in Europe doesn't have much difference in the number of matches for each participant in the UCL at first. But next they will be different because the strength and ability of each team is different. In every league they don't only compete for the knockout round, so that's why the competition will have many matches. Saving energy with some rotations is necessary, it will keep the players fit in long competition matches. So saving energy can be done by changing 10-20 minutes before the time runs out. I think every coach will do that. I don't think it will do much damage, but it is a strategy.
Obviously, rotating players remains the best way to keep the team fit because in one season they will of course play many matches in all competitions. If the team does not rotate, it is very likely that their main players will suffer injuries and of course the injury factor will cause the team to experience bad results. But what is strange is, when a team doesn't make many rotations when playing a mediocre team which is also in the bottom zone of the standings, but the result is still a loss for the seeded team and if that happens, then of course it is a strange result. But indeed, if they have managed to secure the top position and lock it until the end of the season, then maybe they will also play more relaxed and not be too ambitious to win. But if that happens, then of course they also underestimate the opposing team, which of course is not very good for the final result.