I'm not worried about my friends. My friends aren't casuals. What I'm concerned with is how Smash Bros., the series that used to introduce players to many different series and close the gap between being a core gamer and casual gamer, will now be regulated to "that fighting game where you can play as Miis and beat each other up LMAO!", which will overall hurt Smash Bros.'s influence. I never would have tried a Metroid game, or an EarthBound game, or a Fire Emblem game, or a great number of games for that matter, had I not had good exposure to them in Smash Bros. When you potentially allow players to forgo that into just being comparable to that of just another Nintendo Land, that effect is lost. I'm not even kidding when I say some people will legitimately think that Smash Bros. is a spiritual successor to Nintendo Land, either. The casual audience is by far larger than us and most the time much more ignorant in regards to knowing anything about Nintendo.
And it was 8, first of all, in Mario Kart 7, not 12 racers. 12 was on Mario Kart Wii. Second, the fact that there are less potential players is even worse considering that it only takes 2 people playing as Mii for a match to be 50% Mii, as compared to the 6 it would take for the same effect. Scale that to the fact that 5 times out of 8, you'll be racing against other Miis on Mario Kart wifi, and realize that means a lot of matches--2 and a half times out of 4, to be exact--will be nothing but Miis having crouching-and-taunt parties. That sounds like an awful idea entirely. Add onto the fact that some non-casual players will choose Mii for the supposed customization mechanic everybody believes Miis would get, and Smash Bros. will easily turn into being majority about Mii. I know it sounds like I'm over-exaggerating, but again, despite the regular Mario Kart cast being the ones that advertised Mario Kart 7, a lot of the characters were still in minority of usage. People will jump to familiarity in the form of themselves that quickly.
I'm fine with having Miis as playable for the most part. It's just the side-effect they will bring that I absolutely despite, to the point where Miis are one of the characters I'm entirely against simply for that fact.