They'll keep going as long as it sells and leave it a few years then do a rebooted version called New Pokemon, like New Super Mario Brothers if a Gen ever sells too low.
How many years have Zelda, Mario, resident evil, streetfighter, final fantasy, GTA, Metal Gear, Kirby etc been going? If a franchise is sucsessful it'll never stop, look at the James Bond movies for example, even the clunkers sell well enough to continue and every so often it reboots itself with a new Bond that is right for it's time.
Pokemon is the second most successful video games franchise of all time, people will still be playing it in 20 years time even if it's in a form that is almost unrecognisable compared to today. For the moment when they run out of mon ideas they just add new types, which should be good for the next 12 years at least.
