The "create a league" feature is what has kept me hanging around. I'm waiting for the day that enough changes are made to the game to make MFN worthwhile again, at which point I hope to create a league (or maybe join one created by someone else).
I do not think it would be a good idea to require every member of a user created league to have to pay for entry.
In addition, I don't think every league other than user-created leagues should be test leagues. Talk about driving away new players. How frustrating would it be to have built a team who is finally competitive, only to have a change made mid-season that destroys everything you've worked for, and the only alternative is to spend money? This would no longer be a free game, but a free opportunity for you to test out a game.
To me, this entire thread is a non-issue. The system is designed to handle all of this automatically. Nobody is forced to join or stay in a league they don't want to be a part of.
If everyone leaves a league, it will close automatically.
If there are not enough open spots available in the leagues, a new one will be created automatically.
If you can't find one to join that you want to be a part of, create one.