All good points. There's a flip side to the schedule aspect too. Traditional broadcasters also need to fill up slots even if they know they don't have anything great to fill a hole. Which I expect was one of the drivers behind reality TV; you might as well fill the hole with something cheap if you don't expect a scripted show to be at least a reliable audience draw. (And I expect scripted shows are less predictable as well.)

(By "drek," I was mostly thinking of old second-tier and third-tier sitcoms and the like. I'm actually a bit surprised there isn't more of this sort of content; perhaps it's the tension between having lots of content and basically burying your tier-1 content with a bunch of mostly "crap.")