Sakurai has stated on Famitsu that he was aware that people missed certain characters. He's not unaware of the fans' reactions, contrary to what many people believe.
Also, remember that he considered Wolf and Lucas for Melee. Wolf was dismissed as being too similar to Fox at the time (remember that Wolf had only one game and wasn't very fleshed out) and Lucas ended up not replacing Ness because Mother 3 was cancelled back then. They both eventually made their way to Brawl. Obviously, Sakurai considers them important for Smash. A similar story happened with Miis and the Villager and look who made into this game...
Plus, Sakurai dislikes cutting characters, stating that every character has fans. In addition to that, he stated at E3 2013 that he wouldn't outright cut characters out of the way, but there was a possibility that everyone wouldn't return due to time constraints. In other words, the sole reasons for cut characters are factors outside of his own decisions, especially tight schedules.
Given the Wii U's current situation, I highly doubt Sakurai would have been able to delay the game. This gives less time to include everything in the same package but since DLC is possible, scrapping content isn't so much of a big deal as more content can be added later on.
I believe Wolf and Lucas are the first in line for DLC. I'm being skeptical to a degree and not take them as granted, but I'm hopeful for their return. They have a lot going for them and their return would satisfy a lot of fans.