Different to what? That's the only reason I'm ever happy for a player to be retiring. It's also the only reason I've ever been happy for a player to be retiring.
I hate the fact that people, especially sensible people like you, think I want players who once averaged 55 to stop playing the moment they no longer are, even if they're going to be averaging 45 for 20 Tests later on. It's unutterably frustrating, and so completely wrong-end-of-stick.
The truth is, though, times when players are "not up to previous standards but still worth a place" are actually pretty rare. You'd have to be a truly outstanding player (like the above example) for something like this to happen. Should Fleming have played another, say, 15 Tests and averaged, say, 30 in them, that to me would still represent not being worth his place any more, even though it wouldn't be disgraceful.