A second edition is pretty much inevitable economically. At some point, people will get enough of the constant release of new classes, feats and items into the existing system, not even to mention inconsistencies in the current rules and unwanted synergy effects. Sales will taper off as people turn to other, less convoluted systems. And the Paizo staff will want to keep feeding their families.
A new edition will have some die-hards who will stop buying Paizo products altogether, but if the developers manage to make the system substantially better with their new iteration, enough others will keep playing, return or start playing that it will work out.
Pathfinder Unchained pretty obviously is a way for them to probe what kind of changes the fanbase reacts positively to. The developers can gauge the feedback and use it as a core around which to build a new edition of the game.
For main rule system sure but from setting they have room to grow there for another decade. I'm not sure what other books I could possibly need for Pathfinder on rule base point after Unchain and the Occult book. I'd love to see more Golorian hard cover books on different continents.