Some final notes:
The playoffs should still occur for the top 1/2 of the 1st division, and the bottom 1/2 of the bottom division. Even though these teams can't promote or relegate, there can be financial incentives, either through prize money or more revenue from fans. In the case of the bottom division the games might also serve to determine qualification for the next season's cup.
The scheme avoids teams bouncing, either bots promoting and relegating, or active teams relegating then promoting. It also avoids relegating active teams into leagues filled with bots. Since only active teams can promote, a relegated team will end up in a league with other active teams.
Replacement of bots with new owners should be oriented toward maintaining as many leagues in the lower divisions with roughly the same number of teams. For example, let's say that roughly 30 leagues can be filled in D.IV. This means that 2 and sometimes 3 teams will be promoted to D.III. If there are lots of bots in D.III then more teams will be promoted from each league. It is better that the 30 leagues have about the same number of active teams so that each have about the same opportunity to promote.