You seem to misunderstand. Paizo is currently the top-selling RPG. If 5E is a huge success, they just lost their customer base. They could go back to being the small, 3rd-party company they were before Pathfinder, but that's not a decision made easily. The definition of 5E being a huge success is that they pull in a majority of pre-4E players, including Pathfinder.
I don't know that I see that scenario playing out as you envision it. I think with each new edition WotC has made attempts to recapture lapsed D&D players and with limited success. 3.XE seemed to bring a lot of D&Ders back but it's hard to say since it was much harder to gauge the varying communites pre-Internet.
Plus, it sure seems that people tend to either be casual gamers who play from time to time and don't purchase much, staunch gamers who follow a single system, or multi-system gamers.
I think WotC hopes to capture the casual crowd and maybe find ways to increase their involvement by leveraging technology in ways previously unavailable. I think they hope to draw some new gamers to D&D. I think WotC hopes that many of the multi-system crowd will at least check out 5E and add it to their rotation. I also think they hope to keep the WotC 4E D&D loyalists on board while also snagging the WotC 3.0 and 3.5 D&D loyalists who didn't move on to 4E D&D or to Pathfinder.
I think they'd like to recapture some (O)D&D, Basic D&D, 1E and 2E D&D loyalists but this seems like a bigger struggle. They've simply been away too long. Many didn't move forward before, not only because they have in hand the game they love but also, because spending a bunch of money on a new game that won't likely continue being supported beyond a handful of years is one of the reasons they didn't move on in the past. That hasn't changed.
But it seems the majority of Pathfinder gamers are Pathfinder loyalists and likely the toughest sell for 5E. They've got some of the best and the brightest of game design, as ordained by WotC, working on a game they love. They've got a network of 3PP using the OGL to broaden support for their system and probably many still use a number of OGL books from the 3.XE era. I dont think many of them are jumping ship any time soon. There might be a shake up when Paizo finally gets around to revising Pathfinder but that largely depends on the tack that is taken and what is produced. I don't see Paizo as a company that will take their loyalists for granted or, through lack of communication, will take the game in directions that their fanbase will bolt over.
One of the few arrows in WotC's quiver (beyond the brand name) that could help in bringing PF gamers back toward D&D is the OGL. It allows for the possibility that, even if those gamers aren't one hundred percent sold on 5E as it is presented on release, some OGL supplements designed to make the game more suited to their tastes can come from outside. This is why I think it is unlkely that a half-measure like the GSL wouldn't be enough to make a difference. Sure, it could help 3PP sell some adventures and whatnot to 5E gamers, but it isn't going to draw people to 5E in the first place. That's what a full-fledged OGL 5E has a shot at doing.
But they're already messing up. They should have made up their minds regarding the OGL before the announcement. It may well be that they have and are squeemish about saying they have no intention of using the OGL for fear of a backlash. They may believe that by putting off the rejection of the OGL they keep some people on board that would have already written off 5E. But I have to say that, if this is the case, I think this would prove to have been an error much in the same way that dragging out the GSL before 4E was an error. Only time will tell.