Let's say you are stuck in a Tier 5 and you don't get demoted...
what's the point in being stuck in there then? The only thing that can save your MMR would be someone who's willing to play with you and has a higher rank to boost your Hidden-elo. Since at Diamond (my current Tier) the rank differences for duoqing is too small to do that, I suggest that there should be an option to instantly demote after getting stuck. We all know these tryhards that don't want to get demoted, but it would be healthier, for them and for the poor teammates that see the 0/4/0 Riven trying to carry.
tl;dr Being stuck in X 5 and not get demoted only hurts your MMR in the long run, as well as the MMR of your teammates.