As a fairly new player, I think the biggest problem is playing in a league with mostly bots. I like your idea, and I would add something else: reduce the number of leagues in the bottom division if there are a low number of actve teams in that division.
So have half the leagues in he bottom div with 2 promotions from each, or even quarter of thhe leagues with 4 promotion each. All higher divs remain unaffected. If more users sign up then open up more leagues.
Then new players would be in more competitive and nteresting leagues, surely helpimg retention?
Typing on this phone is a nightmare sorry