If you leave a ranked match you lose rating in addition to not getting the reward. If people are leaving that is just the type of person that they are. Incentives probably aren't going to change a douche.
They are going to be holding out the rating needed for the new season so that it may be even higher than 1500 this time around.
As they have not yet posted how ratings/rewards will work for season 2 I am assumed season 1 conditions. For season 1 your rewards were based on your highest rating achieved not the rating you held on the last day.
Now assuming that condition, if I have achieved my necessary rating there is no penalty and no risk to throwing away games in order to complete the daily/weekly.