As cool as it would be to say you were at late 80s or early 90s shows, you'd be kidding yourself if you'd think those were actually "good" shows. They would last 40 minutes, play poorly, and have a lot of technical difficulties. It's fun to watch on youtube now as fans of a big rock band, seeing the band have such a casual relationship with the crowd and everything, but I hardly think I would have enjoyed the show itself. I agree that they might be better NOW in a small venue, but back then, just a promising but still new garage band. And nowadays I'd KILL to be in a small room with Green Day having technical difficulties and all the unpredictability and amateurish, but that's because I'm a huge fan now. That huge fan thing probably wouldn't have existed then, so people wouldn't be like "Awww, how cute, Billie broke the amp!"
I'm pretty happy with having seen them during the 21st century... They've gotten better over time, but they're still young. Right now is definitely the prime time to see these guys, because they won't be young forever. Though Springsteen still does 5 hour shows every night, so here's hoping