Punk kind of ate itself almost as soon as it started. Well...at least punk fans did. Seriously, it became so much about being against "styles" and not idealogies. That's why everyone went skinhead or with military cuts to go against the hippie styles of the generation before. A lot of the people in early punk hated the hippies for falling apart and selling out as soon as the Vietnam War ended. But then the faux anarchy stuff started and people just acted like dicks. Black Flag got so much shit when they grew their hair out.
Don't get me wrong, there were some really great things about the scene: The community, the DIY focus, the amateurish musicians who figured out new ways to make music that ended up being profound. But things became too controlling after a while. Bands who started to have success were called phonies just because they didn't want to live in squalor.
The scene is romanticized a good bit today, but I don't think it was really as great as we think it was. Most of the people involved kind of sound like assholes. The Minutemen seemed like good guys, but a lot of the others were kind of...militant about having it their way. I have trouble listening to The Sex Pistols just because Sid Vicious might have been the biggest douche in music history. Some bands who grew out of the scene (like Sonic Youth, you may not consider them punk but they certainly came from the scene) proved that you can go to a major label without changing who you are as a band.
But as for today, the DIY spirit is alive and well. Fugazi is still making music, and I certainly respect their approach to the business side of things and Ian Mackaye has mellowed out since his Minor Threat days. There are some newer bands like No Age who have that fast-paced and dirty punk sound. Fucked Up are pretty punk in spirit. You've just got to look for it. Even though I think a person can have a genuine punk ethos and become at least a tiny bit mainstream, you're not going to find much of there.