It was easy for Brawl to outsell Melee considering one console had only a 21-22 million install bass, while the Wii has sold over 100 million. Not to mention there was far more hype for Brawl than Melee.
As it stands, I think Smash 4 Wii U is most likely going to sell worse than Brawl, and probably worse than Melee.
But Melee was a release title, and damn, was it a huge improvement over 64. That cannot be denied. Brawl's sales might've had a lot to do with it's (over)hype though yeah. But still. The Wii might've been a better success overall than the GameCube, but fact remains that Brawl was a bigger financial success than Melee.
Ultimately, one character means nothing. Yeah, Sonic created his own sort of hype, and even Snake did it. Then again, that's all cause of the 3rd party and unexpected status. The Ridley fanbase isn't as big as Sonic's for example. Heck, it isn't even as big as Diddy's fanbase in total, cause Diddy had a lot less 'loyal' fans, he was practically seen as a shoo-in, but still massively supported and people where pretty damn happy with his inclusion.
Smash 4 Wii U / 3DS might sell better than Melee / Brawl (Brawl, unlikely perhaps but still) or it might sell worse. I think it'll be a more than successful game though, and people are gonna enjoy it no matter what characters are gonna be in. Unless of coarse they make stupid moves like cutting extremely iconic and popular characters as Wario, MetaKnight, Diddy, Dedede, Peach, Zelda (not Sheik), Marth, Captain Falcon, Ness, Lucario, Mewtwo. Ridley, in the end, doesn't compare to them.
This game will be a success, and yes, there are people who are gonna enjoy the **** out of Wii Fit Trainer, and be HAPPY that she was included. No Ridley is not the end of the world, and in fact, makes his inclusion NEXT TIME all the more likely. Cause seriously, when they add Mewtwo, Dixie, Palutena, King K.Rool, Isaac, Little Mac, a Mario character and Roy / Chrom, who remains? My scenario is: another Mario character, 'new' Fire Emblem character, 'new' Pokémon character, another retro that didn't make it (Mach Rider, Takamaru), Impa most likely if she gets another role, certain cut characters from Brawl to SSB4, and one who might top them all: Ridley.
It's gonna be a hectic time for Ridley fans for sure, but brace yourself for dissapointments please. I myself already have Diddy, so I can tell you that it does feel good to have your absolute favorite character ever make it into your favorite (?) game. So yeah, if Ridley does the same, I hope all the best for you. I know that you, Chrono, at least have been waiting for a long time for this. But here's hoping you'll at least get Roy...