First off, understand the fact that after a game is finished with development, there's a several week period before the game reaches stores and can actually be sold to the public. Between that time, the development team has nothing to do and profits from the game don't come rolling in for around a month or longer. It's during that 4~6 week period that some developers decide to create DLC, which gives them something to use ideas they didn't initially have time for, create additional game content for us to enjoy, and provide them with the funds to work on future games. It's a win-win for all of us when DLC is done right.
Now, apply this to DLC for Smash Bros. Nintendo has shown they're very respectful with DLC, as seen with Fire... well, you know,
that game's DLC. We get additional content like characters and stages that don't make the initial cut. Sakurai and his team have time to focus on improving the game further by balancing characters as long as they need to before releasing them. The masses are kept happy and no longer crawl down Nintendo's necks insisting that we get the next Smash Bros.
now, meaning they don't have to worry about delaying the game in the series in that they have all the time they need. They have more money, which can go into funding Sora's next project, be it a new game or SSB5 in general.
It's a vicious cycle of benefit for both the consumers and the developers. The situation becomes a win-win for both the Smash Bros. team and the Smash Bros. community.
Convinced?