For Bayern you are right if Kompany stays for another season he could make their team even stronger in Germany and maybe help them to do better in Champions League because they will have learned from this season. It is also good for them to use their popularity in America to get more fans and make more money. Winning German league title is big deal and gives them good starting point for future.
Kompany's contract runs until June 30, 2027, so there is still a chance for him to bring Bayern Munich to be much better in the UEFA Champion League. And for the Bundesliga, of course the next target is to keep defending the Bundesliga title.
And moreover in the next season, Bayern Munich has new, stronger ammunition with the arrival of several more players. Moreover, Jonatan Tah, who they have been hoping for since last season to strengthen their back line, has agreed to sign with Bayern Munich.