I doubt they would buy PW but if they do, no the sub only option wont come back. That model is dying out for MMO's, and as we are already a hybrid game it would allow them to make subtle changes in either direction to suit them.
removing the free to play option which has brought in more money and more players would be a stupid decision on their part.
If Blizzard bought PWE and decided to make every one of their games into subs, then I can see it being the biggest mistake that Blizzard will ever make. Countless people would quit and never play the game again. Although we would have to see the percentage of players that pay less than $15 per month and see the savings that would be achieved for getting rid of most of those people in maintenance costs.