Ancelotti has given his best for Real Madrid and he has built this team with a fairly solid framework for the long term so I also think that he will not be likely to be fired from Real Madrid in the near future.
Like before at Barcelo, Guardiola would leave when he had everything to get a new challenge at another team, so it is possible that that will also happen now and there are even rumors circulating about it now.
We still don't know whether to go to Manchester United, but I'm sure that soon Guardiola will leave Manchester City.
If Ten Hag is successful, this will not be his last season, Man United management will definitely keep him if he is successful, but if not, it is not impossible that he will leave midway.
Let's not say he would "never" sign with them, it would be silly to say something like that because more shocking things has happened in football world. I think he would sign if he thinks that it's a worthy task, like if he could turn United into a winning team then he would be considered one of the (if not the) best to ever do it, which could be something he could be interested in.
However, he could of course reject for obvious reasons as well, we just don't know. Plus it is a team with a great potential, doesn't mean they are great, just means that their future potential is great, and Ancelotti is at Real Madrid for a while longer, by the time his contract ends, he may go to United that is a lot better than right now, who knows?