For starters, Luis Enrique is not coming to United, he will have no interest in coming here so I don't know why people are pinning their hopes on this man. He has a good thing going with PSG, and if he did he will end up at City after Guardiola likely leaves come the end of the season.
Secondly, Nagelsmann did not fail at Bayern, his win percentage is 3% lower than Kompany's currently at 71%, he has 4 less wins, 3 more draws and 1 less loss in 2 less games managed. He won the league and the cup in the first season, and then he won the cup again in the second season before the mid season sacking. They were 1 point behind Dortmund in the league and still in with a chance of winning more trophies come the end of the season. In comparison Tuchel's record was 60% and he won the league only because Dortmund royally cocked it up.
I think if you listen to some fans there wasn't a justification in sacking him. Him going on holiday during a break after a loss didn't sit well with higher ups and according to reports he had a bit of a tiff with Sadio Mane. Since then Uli Hoeneß has come out and said it was a mistake and they shouldn't have sacked him.
People need to learn that just because a manager was sacked it doesn't always mean its because they were a failure, sometimes there's more to it. The fact that Nagelsmann could be free at the end of the season should not be a thing, he should still be at Bayern. Its an opportunity nobody should be missing if they need a manager.