Well the Ramones don't sound like the Sex Pistols or Clash and really, those two bands don't sound much like each other. As for Green Day "changing" with American Idiot, they basically have the same sound they always did, just slightly longer songs with a couple uber-long multi part 9 minute plus tracks. That and they are more political, which if anything brings them closer to "true" punk as you kind of infer. Either way, I'd hardly call their new stuff "very progressive". It's the same basic style they've always done, just a little more drawn out with more "grown up" content.

I would also disagree that punk isn't a musical style. True, there was a "statement" aspect to the early portions of the genre that persists today but look at all the amazing bands that came about after the movement got going, calling themselves, "post punk" or whatever (Joy Division, Siouxsie and the Banshees, Depeche Mode, etc). Lots of interesting musical styles were going on at this time, none of which could of been realistically conceived of before the "punk" movement. It pretty much laid the groundwork for "grunge" and what we call modern "alternative" or "indie" rock. This would not of happened if punk music was just about making a statement. From a musical perspective it was simply about getting back to the basics, which is generally not a bad thing. It helped the genre of rock music "reset" itself so it could better move forward with fewer stale ideas being recycled.