The other end of that is that merely going with long serials wouldn't nessecarily be better. I've only sampled Genesis of the Daleks in full, and while pretty awesome there was a certain lagginess to the story. And a rather cheap cliffhanger with Sarah Jane climbing a scaffold. The sort of thing you only see much on reality tv these days: Shocking Event! Break. Less then shocking resolution.
Extended stories would still have static run time increments. Each episode is X length. While its always easy to say a story could benefit from more time, does it HAVE another hour-length program of run time in it?
Not that all of them don't, but I notice fans (of any franchise any media) always seem to call for things to be bigger and more complex... and the flip side is that then you can end up with slow dragging stories weighted down by filler and fluff. Which I don't think is considered all that often.