Thinking of coaches in a similar way to players is actually a very useful analogy because it also helps us see one step further. Coaches, like players, are also not static in a good or bad, black and white kind of way. It's entirely possible to be a good coach one year and then a bad coach a few years later. That kind of thinking almost never enters the debate when talking about coaches, yet when talking about players it is immediately obvious and understanding it is taken for granted. For example, Dele Alli. Clearly a good player at one point and then due to numerous reasons he wasn't cut out for the top level anymore. Players can even go up, down and back up again if they sort themselves out/rediscover lost motivation or adapt their game etc and again it can apply to coaches too.
Ten Hag for example could have been a good coach at Ajax, a bad coach while here this season and there is a chance that he will potentially come good again with a different environment and a fresh start. I do doubt he will have success afterwards, but I wouldn't be so arrogant as to say I can predict the future with certainty, so of course there's a chance.
A counter argument might then be, if you think there is a chance he can be a good coach again then why not give him the chance to turn his fortunes around here? I reckon that turning things around without a change of environment is even more difficult and extremely rare and it would be reckless to hang the hopes of the club on that slim possibility. Again you could say the same for many players who have been at a club for 2 years. Why believe Ten Hag can turn it around but that Casemiro can't? Their length of service at United and their performances at United are almost perfectly aligned, yet absolutely no one has any problem with getting rid of the player. In fact, hanging our midfield hopes on Casemiro rediscovering his form and not bringing in a replacement would be seen as the ridiculous idea.
