why not just vacate your team in the league with 5 unmanaged teams and pick up a vacant team in a different RT league which perhaps only has 1 or 2 vacant spots. If that were done, the league would eventually fold as you suggest, but the other leagues would still be viable and be 24 team leagues.

Personally, I'm in favor of trying to attract new owners and leaving all current leagues intact. I also think going to 20 team leagues just makes the game less attractive and encourages managers to drop out, exacerbating the problem rather than solving it.