"Wouldn't infringers argue this to avoid paying on past infringement (and limit damages to future licensing)?"

I believe they would, but according to what someone reported from the ASHN, Mr. Henneman stated that the 584 had been changed but the 336 had not. So, the 584 infringement going forward only, and the 336 past and present infringement. If that is indeed the case we are looking pretty good.