Before anyone goes on about record sales, yes, we know Green Day has sold a lot of records. So did Britney Spears, doesn't mean she's good.
I don't really have any further arguments other than 21st Century Breakdown was a rehashed American Idiot (and probably a circle jerk with Butch Vig), which was good when it was new. But honestly, like many "political punk bands" that still remain, is there much more you can really bitch about without just repeating what some other shitty band has already said? Christ, even NOFX has toned it down. Billie Joe would like us all to believe the Green Day gospel and his pants get a little wet every time someone reaffirms his belief.
If they gave up the rock opera bullshit on their next album, didn't write "American Idiot Part 3" and did a natural progression like Nimrod was to Warning and didn't care about commercial success like he claimed wasn't an issue with Warning, they'd probably retain a bit more respect from older fans. But, alas, talk is cheap, and money is a flirtatious bitch.
I've only seen them once live a little over a year ago and the show was amazing, but that was probably the last bit of interest I could show in them unless they get out of the pseudopolitical rock opera crap. It's a shame because they wrote some great material in their day.