I am against this, because it has effects to other areas of the game also.
Why?
- Aging
This would mean that all players who play regularly gain an SL between matches 16-20. This "extra" SL will be taken away when the Mid-Season Aging occurs (for a lot of players), so it does not help that much. Also the next "possibilaty is just before the End Of Season Aging and the same will happen. When this would be implemented, the whole aging system is ready for a review.
- TP/CP usage
At this moment managers have to look at the amount of TP/CP they give to a player, because of the aging. A TP or CP to much and more than one is taken away by aging. It will still be a matter of planning.
- Bad season
Teams who have a bad season, get punished for that. Their ratings are not high and have to wait longer before the extra SL is added and that gives them less chance against the other teams leaving an unbalanced league. 
I can see that this would be used to give players a
reward for their good play, but IRL players are not getting better when they get good ratings. They get better by playing matches and good training (well in theory because nowadays nobody gets a chance to train because of the massive amount of matches that have to be played).
If you want to implement a sort of
SL-bonus, why not use an experience bonus instead, linked to minutes played. Each player who has played
lets say 2500 minutes gets an extra SL and it is not counted in the points used, just like the apprentice players. Apprentices are not able to gain this
reward, because they get the seasoning bonus (or they get the
reward and more managers are encouraged to sign any). The player is than rewarded for their increased experience and not for what he gets as rating.
Or change the aging formula, to give age 4 and 5 players a bit more chance of being of any importance in the league. They are now only rescue backup benchwarmers without any future if their curren Sl is below 15. This would leave the whole discussion stranded in the forum.