They really should. It's time to do it. They are losing subscribers and going free to play would attract a whole new group of players to the game.
Players like me who are no longer willing to pay a sub for a game that I would only play once a week would come back and help beef up the numbers on deserted realms.
Many free to play games actually make more than when they had a subscription. Blizz already has a cash shop, with some pretty obscene prices on cosmetic items. All they have to do is add some more and give some perks to people who want to continue paying a subscription: unlimited in game mail, extra rest xp, free charms of fortune each month, whatever.