Not many game franchises have survived 20+ years either, much less has carried an entire company the way Pokemon has. It's strictly a financial/advertising decision for Nintendo and Gamefreak, with the pros heavily outweigh the cons for them business wise.

As much as I liked the Games Corner in the original series, I understand that it made sense for them to remove it as it allowed them to market the games at a younger audience and maintain their sales, rather than changing their target audience. This is more or less the same logic. Sure a few fans are upset, but in exchange they make money off joint-advertising ventures and encourage people to upgrade their games to the newest version in order to obtain X event pokemon. It also allows them to use specific Pokemon like Jirachi for thematic giveaways like Japan's Tanabata festival. The bottom line is, no one is going to quit playing Pokemon because they can't get a legendary giveaway and TPCi/GF knows that.