It'd be nice, but I wouldn't hold your breath.

It seems like WoW might have finally reached its saturation point, meaning the total number of subscribers might have already had its peak in Wrath. Maybe not of course, but it's possible. We all know about the loss of 600,000 players a while back. In any case, it WILL happen eventually if it hasn't already. For the past 6 years profit has been increasing because subscriptions have been increasing. Since ActiBlizzard's mandate is always to seek 'moar profit', without increasing subscriptions they will have to find ways to get more money out of a smaller pool of people. That probably means more premium services, more pet/mount store items, who knows what else.

It has nothing to do with slippery slopes or anything like that. Even if subscriptions were increasing, Blizzard has tried some "premium" subscriptions services and sold in-game items for money, and it worked. People bought it up. It's reasonable to expect a lot more so-called "extras" sold for real money. They're not going to stop going down a road that's making them a ton of money.