i doubt that's the intent... so, enjoy it while it lasts
sounds like a good idea each year is to fire one minor league coach and see which high-end coaches are interested. (if one isn't leaving.)
an exploit if you do it to an abusive scale, but like you said, you won a bunch of titles with them. rarely do coaches leave with such success, unless they see a dumpster fire on the horizon. i wouldn't feel scummy for doing it with my current coaches, if they normally re-sign extensions, but i forgot one year and ended up in same situation as you.
i don't think pesonality conflicts with the GM are modeled in ootp? could look into other stuff like relationships with the players, but that's all a guess as to whether it is actually important to re-signing a coach.