With this change, if you constantly run M+ and are consistently timing keys, then you are being rewarded for your good play with higher level keys. Those people that just did one key a week, timed or not, or only played M+ when the weekly affixes were easy are now pressured into pushing themselves more in order to reap the benefits.
Anything that forces people to play better is good for the game. Sitting back and resting on your laurels is no way to go about things.