This already been discussed before, the problem is that If you would be relegated, you could buy a team full of 5* players near the 6th star in the last days of season, and in the next one you would one level lower with a team full of scouts near the 7th star.
You might think this is ridiculous, but If some guys sell all there players at the end of the season and buy 3* players to get easyer CUP draws! you can bet someone would do this.