I was unhappy Pep didn't make changes and came here to write just that. But we don't know that (tactical and personnel) changes would have made us better.
Imo, Pep didn't make changes (earlier) because he trusts Gundogan and KDB. Also, Foden had scored (and is scoring much more often than Grealish and Mahrez anyway) and Bernardo is too important. Gundogan has scored crucial goals and is captain for a reason, and KDB can produce something out of nothing even when he is poor. KDB was poor vs Villa in *that* game, but assisted the winner for...Gundogan. That's why, I think, Pep decided against subbing them.
In hindsight, Pep was wrong today, we couldn't be worse off with earlier subs. As you say, we were getting worse in the 2nd half, Pep failed to change that. He might have made bad mistakes indeed. Nobody can be absolutely certain about the right decisions at the time, though, especially people who have never managed at professional level. It's not about being uncritical or cultish, it's about knowing one's limitations and the nature of the beast (football, that is).
Imagine that Pep had subbed off Gundo, KDB and Foden for Alvarez, Grealish and Mahrez and we lost. He would have had a shocker then for subbing in Grealish who has 1 goal for Foden or KDB who are far more productive...Maybe there are other things, too. Pep wants Gundogan to stay...