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Unless Satoshi Tajiri is discovered to be a serial killer or a pedophile or something I don't think the Pokemon brand will die in the forseeable future. It's only becoming more profitable, it's surpassed Mario as the most widely-known household name in video games, it has appeal with kids and adults and it's extremely popular in Japan and America. To stop now would be foolish and would reflect poorly on Nintendo.
Barring extreme circumstances like Pokemon being involved in some irredeemable controversy or the entire market collectively losing interest at once, Pokemon will stay afloat as long as it's properly managed. Even if Nintendo goes under (which might happen within our lifetimes) someone will buy the Pokemon license and if they do a good job with it then Pokemon will continue to be popular. I suppose some other 'mon franchise could come along and eclipse it in popularity, but that seems extremely unlikely at this point.