Honestly, I've said it for a while now that Smackdown just needs to be scrapped completely, it's past it's sell by date and has become stale, even the hardcore fans find it a chore watching this show now, they should just scrap smackdown and replace it with something completely new I think.
By that reckoning you could say why don't WWE scrap house shows, they aren't on TV and arenas don't sell out. As long as a TV channel are willing to pay for SD and people come to the shows there's no reason to scrap it, they're making money, not to mention Superstars and Main Event are recorded at SD, they'd lose the money from those shows as well.
Because it makes money and expands the visibility of the WWE brand.
Wrestling, because it's a self-sustaining product, is easy to air on TV. As long as it makes the network money, it makes them happy.
It would take the network canceling Smackdown for it to go away. Even when they have, they've simply changed networks. Still, it's been getting smaller and smaller with lower and lower stakes each time they've changed channels. I think it won't last another 10 years.