Considering the amount of thought and effort that seems apparent in all of the extra modes and such, leaving them out probably would have shaved off three months... And that's being really generous. Most of the trophies are just pre-posed models of charcters that are already in the game, and of the ones that aren't, about 80% are just the models and textures jacked straight from another source. With years of modeling experience backing you up, and undoubtedly a large team, you'd be impressed with how fast you can crank out things like trophies, you know?
Since all of the game modes that focus on fighting use the game's base mechanics, it isn't actually much time at all to include Classic (randomly generate groups of fighters and stages for that group of fighters to play on), All Star (randomly generate a stage from a pool of stages for predetermined, unlocked characters to fight on), or Master and Crazy Orders (randomly generate characters, game mode, stage, and gimmicks). I would be beyond surprised if all of those game modes took more than a week, really.
Then we have Smash Tour and Smash Run. Smash Run already uses the basics of the game and just includes Subspace Emissary style enemies and area warps, which would mean the barebones code for handling that was already hanging around. I can speak from experience that coding a powerup system like it uses is really easy, so Smash Run, even though it actually has a little bit of love put into it, probably took less time than Smash Tour... but hey, the guys working on the next ten Mario Party games were probably across the street and very willing to help, right?
All of the time that goes into Smash can pretty fortunately be attributed to what the game is actually about. I'm guilty of complaining about the extra modes and all of the bells and whistles that no one ever asks for, but realistically, without that stuff, they would have been able to get Mewtwo in before launch and that's about it... Unless you're considering custom moves extra stuff, in which case they could have cut that garbage and all of the DLC we have right now aside from Mario Maker's stage would have come with the game.
