I think that was directed at me, and if so I actually agree, and made the exact same point on another film thread. But I would still refute the notion that the 70's are only regarded as such an innovative period in American cinema because we're only remembering the good bits.
It's period that fell between the disintegration of the last vestiges of the studio system and the invention of the high concept event movie.
For a decade where the old guard struggled to acclimatise, a wave of young 'auter' directors managed to sneak into the mainstream and make a series of films that aspired to a purity of purpose.
Some worked to brilliant effect, others inevitably fell into a mire of self indulgence, but I don't think Hollywood has been as consistently interesting since, and probably won't be again.
The comment wasn't entirely directed at you, but more at the general sentiments in the board.
I agree that it is entirely possible that the 70s stand out as particularly innovative, because you aren't the first to say that. I don't know enough about 70s cinema to make a judgment, though. But the larger political, social and institutional climate does contribute to the boom and bust of film-making at large.
Speaking of boom and busts, I think in 10-15 years' time we will be seeing a new boom in cinema, led primarily by the indies. With the advent of networking and the twitter age, word-of-mouth is becoming progressively more and more important. That, and the public is bound to become disillusioned (which I feel they are already beginning the process of). With filmmaking becoming cheaper and cheaper, indies are able to realize their artistic vision more and more. 99% will be crap, but due to the sheer number of films made there will be lots of gold, too.
All we need is for the indie crowd to start really learning and adapting to how internet networking works, and to use it to full advantage to rival the major studios. That, plus a move towards making "indie" a kind of brand, thus making it a selling point. When the day comes when the movie crowd consistently thinks "let's go watch that, the big studios haven't touched it!" the revolution will realize its full force.
JMHO, and liable to be complete BS as I have practically zero knowledge about how the film industry works. Consider this verbal diarrhea.