Well, it's impossible for it to go on forever. Like all franchises it will eventually die. Sure people may still go on talking about it, and there'll be a few forums online still active like this one, but as soon as Nintendo gets less money from it, they'll stop with the games and come up with a new trend to take its place.
Personally, I'd guess this could happen by the end of Gen 5 (and by "end", I mean after they've made a trio of handheld games, a few spin-offs, and one or two console games). They got too much money off D/P to end it now, but who knows what'll happen next time around? Maybe it will keep going. Maybe not. After all, it's getting a little bit old, you have to admit. Nintendo's concern isn't with us anymore. It's with the younger kids who buy/will buy more of it. So, no matter how much we keep playing/watching/hoping, when the 7 though 11 year olds get bored, Pokemon goes down the drain. At least, that's what I think will happen.