1. When you don't get promoted you are more likely to have a stronger squad than those that are promoted into you league, without you even having to do anything.
2. You have an advantage in terms of strength.
3. Unless you are paired with a promoted team who is already stronger than you, you would probably run away with the league.
4. You will however lose some income since you do not play CL and have to rely on a good cup run to collect money.
5. During the season you tank, you can train your players to be better.

Experienced tankers and back me up here...