Personally I don't feel like the franchise has another 3-4 gens of success left in it.
I think at some point (which me may already have passed) it will become so repetitive that the original fans of the series begin to lose interest entirely, opting to quit when they still have fun and unspoilt memories of the franchise.
However, while that indeed happens, every generation serves to drag in new fans to the series. Those fans will stick around for a generation or two, before they eventually outgrow it. But by the time they quit, new ones come to take their place, and so on.
Consider the great American comic book heritage, for instance. The characters are 70+ years old, yet still attract new fans. Those who read the originals might think the direction the franchise has taken and the choices done by the executives have completely screwed the entire spirit of the series, while younger readers are unable to identify with the original series their grandparents loved. Either way, the franchise survives and is popular. Pokémon may still have decades ahead of it before discontinuation - even though that might mean a gritty reboot now and then.