The first thing I’ll say is that I’ve not been a big fan of Seinfeld, and it more than anything else watching it doesn’t make me feel good.
The second is that if the content of a show doesn’t resonate with viewers, it doesn’t last very long. I would presume that the resonance is a reflection of reality, if not in a literal sense then definitely in an emotional sense.
Third, television doesn’t exist without advertisers. Advertisers prey on two motivations: fear and greed. I would say above all that both shows’ primary purpose was to motivate people to buy things (and to perpetuate fear and greed). Interestingly, the writers of “Leave it to Beaver” were former ad men.
Last, I think the values a society embraces also follow a yin/yang pattern, whether it’s on TV or in real life, and I’d argue, the shifts are the products of economics and politics (colonialism, industrial revolution, WWI, roaring 20s, depression, WWII, postwar boom, red scare, sexual revolution, Vietnam, war on drugs, fall of communism, etc.) One could argue that the sexual revolution was a backlash against the values of the 50s and that the revolution led to a backlash against feminism in the 80s.
“Leave it to Beaver” aired from 1957–63, which leads me to believe that there was a lag from the onset of the economic suburban trend until the show aired, but that once the sexual revolution took hold in the public discourse it was no longer feasible to keep it on the air. I have a harder time making the argument for Seinfeld, except to say that it is a consequence of postmodernism (a show about nothing) and like “Friends” reflects the family-less migration to urban centers among younger, career-minded adults (again if not in reality then emotionally). Maybe, too, the show reflects the emotions of the first real era of downsizing, layoffs, and globalization where hard work and loyalty led to a pinkslip. Other than absurdity, frustration, and a whack of impotence, what’s the lesson to learn in the reality of downsizing? Over what other than the petty could the collective consciousness of that era really exert some control?
I guess I think television primarily reflects reality, but also serves to articulate and codify it after it has happened. It (and its advertisers) are interested in highlighting and to an extent perpetuating the emotional aspects of trends to capitalize on the public consciousness. (So I guess the answer to your last question is yes, until enough people start focusing on something else.)