Or people could try to share power with fellow players, particularly newer ones, instead of playing margrave/duke/kings. Might help keep some newer players around to get them involved.
It'd be problematic if 5% of a realm's nobles hold 75% of the titles. However, that isn't the case. Almost everyone, everywhere, is a lord, or has some title of some sort. Being a ruler/duke isn't really hogging titles all that much, especially considering that the ruler wouldn't have created that duchy to begin with otherwise.