I doubt it would "flop", but I agree in the sense that the sales won't be
nearly as high if Shulk wasn't included. Compared to Xenoblade Chronicles when it started out, X is already receiving some attention globally, so I'm sure people will get into it just because it 'looks cool' and plenty of others will get it that enjoyed Xenoblade Chronicles. Shulk's inclusion would kind of remind of how things were with Marth and Roy before Fire Emblem reached the states. Fire Emblem has reached its peak with Awakening...or so we thought. With Robin and Lucina's inclusion and FE reaching a whopping 4 reps, I'm sure FE fandom will continue to grow, boosting Awakening's sales even more as well as the upcoming SMT x FE title. Xenoblade Chronicles talk has been spreading around like wildfire and more people are getting hip to the game, loving it, and coming to support Shulk. so getting into Smash would just let things reach maximum hype. Plenty of people that weren't aware of Xenoblade before will surely backtrack to play his game as well as get into X because of the love I'm sure they'll have for the character.
In-game advertisement is a helluva drug.
Roy is a Smash favorite for a lot of people simply because he has a catchy chant from the audience and his blade emits flames. Outside of Smash, Roy isn't even cared for much. Outside of Smash, Shulk is awesome. Inside of Smash, Shulk would be awesome. His inclusion will be monumental for the future.
P.S. - My power went out when I was trying to respond earlier. So this is late