Then they should have prepared the game to serve such a role (I.E. establish something similar to Fight Money, a set schedule of character releases, etc.). Take Street Fighter V for example, Capcom has plans to go as long as 5 year for supporting the game, that's a game that will be a service. It had a rocky start, sure, but Capcom has a clear-cut plan. Smash 4, as far as we know, doesn't have a similar plan behind it for long-term support. And even if it did, we're already 2 years into the game's life. It's only a matter of time until interest in the game dies down, regardless of how much life Nintendo tries to pump into the game. As great of a game as it is, Smash 4 can't last forever. And Smash 4 (again, as great as it is) is nowhere near perfect.Smash will likely become a service game so I can see plenty of characters joining the game. Even characters Sakurai wanted before but never had the time to make real. I definitely see Ice Climbers returning. There was also a time he was interested in Plusle and Minun too, so that would be cool.
As for Brawl still selling, don't forget the anomaly known as the Wii, in addition to sales to mainly play Project M (granted, it's nowhere near the majority, but it's something).