They know the game has a lot of demand. Some first party games release a new game every year. Nintendo releases a new Smash Bros every new console as it seems.
Though, fans had been demanding Nintendo to pursue DLC for future games as it will up the replay value for some games back in the PS3 and Xbox 360 era. They had experimented with it a bit for the 3DS and Wii U with games such as Pikmin 3 and Fire Emblem Awakening.
I think the DLCS for Fire Emblem and Pikmin 3 were cheap and was worth every dollar.
Regarding PlayStation All Stars, I did not like the game because you couldn't unlock new characters through gameplay. All new characters were in DLCS, 4 dollars for 2 characters I believe. I know Smash Bros has unlockable characters in gameplay and now DLC characters. I'm hoping they don't go with a ridiculous requirement.
To be fair, since you bought the 3DS version for your sister and you're going to get the Wii U version, that means that you and your sister can both get Mewtwo for free :P