i'm personally hoping for a spinoff of some kind after IoA/CT, but who knows. it's something worth wondering.
Remember that the only spinoff Game Freak has made is Quest, which almost no one wants more of.
People balking at another $30 expansion would probably happen (in fact has already happened) but it'd be hilarious considering you know
the alternativel is a full $60 game anyway
People who buy all the games new get a better deal, and so do people who buy only the first games of each generation, but people who buy used or who skip the first pair of games to wait for the third versions get a worse deal. People who sell their games get a worse deal also.
(Aside: I still find it so hard to believe that a
majority of all casual Pokemon players get only the first games of a generation and nothing else, according to the sales data. That is, these are the people who bought Ruby/Sapphire and Diamond/Pearl, but skipped both Emerald and Platinum, and they outnumber all Emerald/Platinum buyers. The appeal of "oh look shiny new generation with 100+ more new Pokemon" really does sell more than superior gameplay, sadly.)