Some people will pay though, the cost will be outweighed by having to move to another service and move all your pics. Which could be in the thousands for some people. But I can't see it being enough people to make it worthwhile for PB. As you said, I expect it to tank or to once again revamp their business model.
I dunno, I know many of forums and websites that had thousands of images reliant on PB, but it's a simple transfer to places like St.ash. Otherwise it creates a knockon effect where forums would have to start using subscriptions or run a lot more invasive ads.
Imagine KMC being a subscription only forum, or full to the gills with invasive video and popup ads....
I'd say "f@ck that!"