I am uncertain as to the cost/benefit projections for doing that.
Yes, they get to bump subscriptions a bit. But the game would effectively go f2p in the process. And anyone that already pays for both, will not add to any numbers.
I don't know the current player count for WoW. Only MS can estimate as to it's potential benefit.
My money is on them just migrating everything to their storefronts to begin with and then waiting to see what effect it'll have.