Well if you look at the present time there are hardly any demons around, so most of them must die out in the next few hundred years from the feudal era. So even if he stays a demon or half demon in his time, just like sesshomaru, they probably would get killed eventually. Plus even as a half demon he probably will out live kagome cause deomons live longer, sesshomaru being well over a hundred years looking the same from when inyuasha was born. He was asleep for fifty years and looked the same when he woke. What happens when kagome is old and he still looks young.