I agree that American network tv has gone downhill in the last decade.
I think tv peaked during the early to mid-90s, with the late 80s being a runup to the peak. Most of my favourite tv shows aired during that time, all of wich were fairly popular shows:
Cheers (1982-1993)
The Cosby Show (1984-1992)
Full House (1987-1995)
The Wonder Years (1988-1993)
America's Funniest Videos (1989-present)
Seinfeld (1990-1998)
Home Improvement (1991-1999)
Fraiser (1993-2004)
Friends (1994-2004)
In the late 90s there was a move towards angry humour on most tv sitcoms. New tv sitcoms failed to match the quality of those from the late 80s and early 90s. Eventually, all the remaining sitcoms from the 80s and early 90s era ended.
Going into the 21st century, there was a move away from sitcoms, and towards reality tv and crime shows. This shift reflected a move towards more conservative values on tv. There were new sitcoms, but most focused on the well worn family concept, and relied on angry, bigoted humour. There has been somewhat of a shift away from crime shows in the last couple of years, but the reality shows have continued unabated. The networks like reality shows because they don't have to pay actors to star in the shows, and they don't have to pay writers on these shows as much.
The only current shows on American network tv that I watch are:
Americas Funniest Videos
CSI
Gilmore Girls
The New Adventures of Old Christine