If new teams are placed in the lowest division, and steadily added across the division you will have new teams that were added within days if not minutes of each other. They could all be started out with a 0-0 record. Even though their initial season is short, it is better than placing 16 new teams in a division, and giving one team 16 wins and another 3 based on the performance of some bot before they arrived.
In each league, there will be some who who really get into the game, some who play a few weeks and stop logginng in, some who look at one game, and some who won't even log in after they sign up. Those who aren't active will tend to slide to the bottom of the standings, and once they go long enough, they can be made a bot and slid to the bottom of the standings.
If BB gets to the final league in the bottom division, they can start over at the first league in the division. By that time there should be a large number of teams that will have become inactive. And then BB can then replace them all within a few days of each other, with 0-0 records. They wild simply form a 2nd group of teams behind the veterans. All teams wouild then start the next season 0-0.
If a team goes bot during the playoffs, there is not too much you can do about it. If they happen to win the league, you can not promote them. And there can be rare cases where a replacement team might win the the championship team and promote, but that will be relatively rare.