It might not be fair being stuck in a tough league, but it seems to make you a better BB player. Look at some of the teams who promote out of these tough leagues. They end up promoting to II right away because of how strong they are.
But that's not the point. The intended way to promotion is winning the league championship. The fact that there is cleanup due to turnover is necessary, but not a given.
Everyone wants to promote. I see threads where people ask if they will be able to promote based on record. All that hope in the cleanup is misplaced. At some cutoff line, the almost made it team will miss promotion, and say, I deserved to promote!
It's a false sense of entitlement. If you looked at the last team in and the first team out, would the one who promoted really be that much better than the one that didn't?