I said this years ago ( http://gamrconnect.vgchartz.com/thread.php?id=181277&page=1 ) , and talked about how franchises like Mass Effect and Call of Duty would also reach legendary status if only one (or two) entries came out each gen.
Its not magic, its perceived value. If a game comes out every year people think less of it, regardless of its quality. If a game is multi-plat, same thing. "OMG its also on PC" third party games would be treated like the second coming if they became truly platform exclusive too.
That's why Nintendo (and its fans) hate the concept of going third party because then the IPs perceived value would go down and they wouldn't be able to make bold and ridiculous claims online.