Its not just a weight issue but also a frame issue. These new welterweights like Shavkat, Khamzat, Garry, Edwards are much taller, are just as heavy without being bulky, and have about an equally long reach as GSP, because they have a naturally bigger frame.
GSP also enjoyed a significant strength advantage over his opponents, would you comfortably say that GSP would have a big strength advantage over guys like Shavkat, Khamzat, Usman?
I think the whole idea that the new generation has gotten much better is blown way out of proportion, but I do think that welterweight specifically for some reason is seeing an influx of guys with MW frames somehow making WW consistently. There was a time when Condit was the tall guy of the division at like 6'1/maybeee 6'2, while looking kinda scrawny, and now we have a guy like Garry whos like 6'3 and makes WW without looking emeciated.