I'd still like to know, how does the upstart team with no chance to stick in their new league not lose fan support the following season?
Instead of preventing tanking, you should have rewarded competing. Stajan is a good example of this, but there are dozens, probably hundreds every season - the well-managed team that wins its weak league and promotes into a strong league, where their arena is too small and the players too weak, and inevitably gets relegated despite competing every game against far stronger and more established competition.
In real life promotion/relegation scenarios, fans understand the reality of these situations and stand with their team. In BB, fans deem the season a failure and doom the team to financial ruin. This is what made tanking financially feasible, of course - if you're going to take the hit, you might as well take as much money as you can with you.
I feel like this idea (preventing tankers for the good of the league) is halfway developed, and should not have been implemented until the other half (rewarding competitors and giving them a road to profitability) was complete.