I think there's enough 1st party games on GCN that it would keep people interested, & looking past that, while not as good as the PS2, 3rd party support is still pretty decent. Beyond good & evil Sonic Adventure/heroes/riders Super Monkey Ball Billy Hatcher MADAGASCAR??

I think if they were really struggling to fill out the library they'd have no issues convincing their 3rd party partners. The problem arises more when comparing to their recent HD remasters/remakes. Wind Waker Twilight princess Paper Mario ttyd Pikmin 1/2