Paul Williams would never be a Welterweight under those rules, so no. The same goes for most Welterweights.
Its the same in most divisions, a fighter fighting at 130 isnt naturally a 130 fighter its the weight he can comfortably get down too for weigh in day without draining himself then straight after weigh in pile the punds back on. There are exceptions of course but the majority of fighters aren't within the weight bracket when it comes too fight time.
I don't think he can honestly make welter anymore under the current weigh-in, honestly. He didn't even want to come down to 154 for a title fight with Martinez.
Because fighters will continue to rehydrate themselves to make the weights they do now. They won't get the chance to rehydrate and there will be far more serious injuries and deaths.
i agree that they will still try and cut to the lowest weight possible and will not have time to rehydrate.