Well, if a second division team dominates the league and gets promoted it doesn't mean it will dominate the first division as well. Their players will still be from second division and not on par with the first division players.
Higher league, higher difficulty and hence you have worse players.
It happened occasionally in real football that a newly promoted team will also dominate the league and the same can happen here if you buy and manage the right players. I won 2 leagues in a row without buying new players, but the second one has been a lot more difficult.

I know it pisses you off, it does it with me as well even after 2 years of playing. That's why I intensified the training sessions so that young and scout player will keep up with the promotions. Of course you will always need a few new players to fill some gaps.
Just like you lost a star you can still regain it.

I could keep 3-4 players for several seasons, even up to 5 or 6 and they always gained at least one star every season, 2 when they were young. I sold one of them this season because he didn't score anymore.