It happened in the 80s, too. These things have been fairly cyclical for them.
ABC was in decline after Home Improvement and the TGIF era ended until Lost, Desperate Housewives and Grey's Anatomy (Millionaire didn't really do much).
But while CBS was able to brand off of those shows, it was harder for ABC to do the same with its hits and is now trying to build its sitcom brand again.
We'll see if NBC can do this.