"If you have an Xbox and need a game, and you could install a Nintendo game on your Xbox, then Xbox games and Nintendo games would be the same market. But since

"If you have an Xbox and need a game, and you could install a Nintendo game on your Xbox, then Xbox games and Nintendo games would be the same market. But since you can't, they're not." Sorry, that makes no sense. Microsoft, Sony, and Nintendo all compete in the market of video games. They are not competing in three separate markets of "video games for Xbox", "video games for Playstations", and "video games for Nintendos". Nintendo games are substitutes for Xbox games, even though you can't take a Nintendo game and directly plug it into an Xbox. As another commenter put it, you're gerrymandering the definition of the market to suit your argument.