Seasonly would cause a run for new staff at the start of each new season and the prices would skyrocket.
And this would also be easily avoidable by firing your trainer at end of the season, train FT or ST for two weeks and wait for buying frenzy to finish, then buy a new trainer.
Sometimes from a point of view of game mechanism, we have to stay away of the reality.
Sorry, not every country has 4 divisions. Last season there were 2 or 3 new users who ended up in my league. And I don't think I see to tell you the rest of the story.