The show is from the perspective not just of the ad men characters, but the pre-boomer age they came from. It's natural that only the most ridiculous aspects of hippie culture came to the vantage point we see in the show. But it's also portrayed as a dying age of decadence and fault, on the verge of being torn down by the rise of the baby boomer generation. The ad industry adapted to change, with only the younger cadre of Sterling Cooper (as well as Don, the chameleon that he is) were equipped to make that change happen and follow it through. All that culminates in the Coke ad, which is completely and blatantly targeted at the exploding -- and newly cynical -- boomer generation.
Keep in mind, though, that even that generation quickly betrayed the counterculture they helped create. They expanded the suburban lifestyle to degrees Don's generation could only dream of, and eventually they embraced Thatcherism and Reaganomics and began dismantling the social safety net that served them so well. To some extent, the left of the 60s deserves some ridicule for their failure to follow through, never mind the degree to which they were duped into thinking the totalitarian communist regimes were actually building a utopia.
It's a cynical ending, to be sure. But I think it's actually a pretty earned cynicism. Honestly, the 70s are where the dreams of the hippies went to die, and then the 80s stomped on the corpse.