Are the fighters today stronger or are they all glass chinned?

Yeah, but why the parallel evolution in other non-mainstream sports like running/swimming/weight lifting? Those are certainly not marquee sports but the improvements in those sports are very pronounced.

Plus I feel that this is a very american perspective. American Boxing might be dying but World Boxing is very very strong at the moment. (Case in point: the Heavyweight Champ is from Ukraine, Cubans are coming out of the woodwork, mexico has been dominant in the lower weights, and the consensus pound for pound 1 (or 2 whichever way you swing) is Asian.)