Has it actually been confirmed that it's coming out this year? And if it does and it is being built on Smash 4's engine, it goes back to my other comment. They can't do that for every single possible Smash game afterwards. Let's say they keep making Smash games for the rest of our lives. Do people really expect Smash 9, probably about thirty years from now, to have 120+ playable characters? It's just not feasible and it's not healthy for the community, as the roster has become so large that entire sections of it will be completely ignored, the player who now feels so overwhelmed with so many options, they may never become a master of any character if they keep trying to try each one, or the game, which now has to incorporate elements (stages, trophies, music, etc.) for all of those characters. It becomes a mess. If the series truly goes on for many years and has many more installments, expect to see many characters come and go over the years with a core group of 20 or so characters that are staples for every game.
There's also development time. While it's easier to reuse elements from characters that already existed in the previous game, but it's still not a simple copy-and-paste. It still takes time to texture the models and balance the moves. If you're doing this for 50+ veterans the time still adds up, and will continue to the more you pile onto the roster. It's resources that could be spent making some fresh new game modes or amazing stages, making the game the best and most beautiful it can possibly be rather than cramming as many characters into the game as possible for the sake of having as many characters in the game as possible because some people can't stand the thought of losing characters they never touch anyway.