I think there are two generations right now vying for pins, which are driving demand.
Boomers, who are late middle age or retiring. Their friends bought Harleys and they bought pins.
Gen-x grew up in the last hurrah of the arcades, and the last heyday of pinball. Now Gen-x is buying houses, raising families and wondering what to do with the basement.
Neither generation is going away anytime soon so I think the collector market will be around for another couple decades before pinball becomes the next pitch 'n bat. One caveat, though, is if some miraculous "game changer" occurs in pinball, making previous games less valuable.