Make it impossible to kill anything and hence impossible to get any drops. Effectivly ruin the game for anyone who likes PvE and lighten the load on your servers. Afterall, this will reduce cost since you aren't getting a monthly fee and you already have our $40-$50.
Create an update that reintroduces fun PvE so that you can once again market toward this sector of the community. Get them to pay $40-$50 for the game and then after your customer base is large enough and you've made enough initial profit, screw up PvE again in the method described above.
Rinse and repeat until no one is willing to buy an expansion or play PvE at all.
