There is a reason Mearsheimer is part of an IR school of thought called "Realists." This analysis doesn't focus much on self-determinism or international law because it assumes that these concepts don't have much influence. I also think that it's regrettable, but people making foreign policy calls should be aware of this line of thinking.

For what it's worth, what Mearsheimer is saying here about NATO expansion being destabilizing is pretty much exactly what Putin is saying in his various speeches.