Why characters will get cut is the same reason others have already been cut...
-Low Priority (Clones, Mewtwo, Squirtle, Ivysaur, etc and almost jigglypuff in pretty much every game)
-Technical Limitations (Ice Climbers, may not be as much of an issue anymore due to increasing hardware and power)
-Successors (Young Link->Toon Link, still unconfirmed if this is the case keep in mind, but I think it more than likely is)
Thing is, for those against cuts, you have to remember something...new characters come at the expense of cuts...with each installment of Smash the rosters keep getting bigger and bigger; so it'll be harder and harder to replicate AND add-on to it in the future since that requires more and more work...especially with Smash that has a huge focus on content...
hence why characters become low priority, which usually falls to clone characters since at the time they were added more for convenience at the time...
Now for a port+more content, cuts are likely not going to happen since it's literally just porting the data over and spending more time adding on to it...but for a new game is a different story entirely...