When I say stop watching TV, I don't mean stop doing sedentary things. I specifically mean stop watching TV. You can do almost anything else and I sincerely believe that you'll be better for it. Read a book, exercise, learn to cook, program, help your kids with homework, play video games, go out with friends...anything is better than watching TV.
:) Clearly you are not a hard-core gamer or you don't have any friends who spends 200~ hours on a MMORPG per month. Although I generally agree with that TV argument.
Have you played an MMORPG? I've done both (200 hours in a month of MMO, 200 hours in a month of TV), and I think the time spent on the MMORPG was far more of a waste.
It probably depends, but I'll add my 2 cents. Never done 200 hours of either in a month (really? 200 hours? that's insane), but after I went on my Dexter marathon, at least I'd seen Dexter. I still reference it all the time. The hours I spent on WoW were... well, I don't remember. That's probably your point.
You should suggest and encourage them towards new activities, rather than trying to get them to stop one activity. It's psychologically much more effective.