You are absolutely correct. Your post is referring to the mental / psychological issues related to the rotation process -which is one of the most complicated ones in modern football. There is a series of variables involved and you have to deal with them them correctly, most of them on the training ground, others in term of man management. Without hesitation, Guardiola's City presents the most successful application I have ever seen -which had a huge impact to the team's achievements, especially in the last two (by definition complicated) seasons. Not without problems though, they will indeed occur, practically it's deterministic: To use your language, a certain senior player did not accept that policy last season. It fucked his head -which had a negative influence to his performance and to the team as a whole. The difference with Guardiola is that he loves his players, he will never throw them under the bus. He left the door open for the player in question (like he does for everyone), and things are significantly better this season in terms of his contribution to the team's effort...
We also need to identify another issue here. This season we suffered a lot of "small" injuries. There were players close to hitting top form (some of them having left their injuries behind) when suddenly they had to be taken off for treatment, for a period of like 1, or 2, or 3 or more weeks. Which is an annoying thing. Because when they recover, they will have to get match fit again. And the only way to achieve that is through game time. Phil, Jack, Ilkay, Kevin, Gabby and others, have all experienced this type of situation...
With everything on the line, Pep had to find ways so as to smoothly incorporate these players into the team's proceedings again, while at the same time avoiding all possible costs. Either by starting them in the initial lineup (having assessed the risk involved) or through substitutions. Making sure that both objectives would be fulfilled: Players would get the necessary minutes AND the team would keep on winning...
When such a thing occurs, some people fail to understand its importance (the moment it occurs). And when it starts paying off with players gradually finding their form again, like after 1, or 2, or 3 games, the same people fail to understand how carefully the manager has planned this process. It's not like things just happen, and it's certainly not fucking coincidence. Everything is planned and taken care of meticulously...
As for the youngsters, what can I say. You can only hope that talented boys out there will be lucky enough to find somebody to treat them and take care of them the way Pep has with a certain Phil or a certain Cole, for example. It's education at the highest level (the term Danilo and others have used in interviews so as to describe their experience working under Guardiola's regime) involving everything related, tactics, technique, mental maturity etc. Most of all, I would say interaction within a team that operates in the framework of a specific strategy. The man knows what to do, how to do it, when is the perfect timing to do it...