High MMR can stop rank progression.
I'm a Silver 1 player who is not allowed to que with Diamonds yet I am matched up against them. I'm also matched up with "plat" players who have low MMR. So essentially I had high MMR so I'm put up against good players but teamed up with "bad" players for their rank. This is a really bad system and needs to be stopped. Only match people within your rank bracket or the next. The fact that I can play against a Diamond but not with one is absolutely ridiculous and stops high MMR players stuck in Silver and Gold like myself, to get screwed over.