What’s wrong with anecdotal evidence is that it’s well… Anecdotal. It’s not indicative of anything. I could just as easily come back and say “yeah I played KI with one of my buddies a few weeks ago and he seemed to really enjoy it, so I see a huge future for KI with tons of players that love the game and a thriving community.”
I mean, there’s nothing to extrapolate off of there. As for zero fifths of the MK fanbase playing MK now, I don’t quite see how that’s the case. I also don’t think that only one fifth of the people that bought MKX could be classified as “hardcore.”
To me, there are different definitions of that term. For some, it may be going to tournaments, following the big name players, etc and if that’s you, then that’s fine. That’s a valuable part of the community.
There are also people that follow a game on sites like this, they play the crap out of the game, online or off, and they get a lot of enjoyment over a long period of time. Maybe they don’t go to tournaments, but they also don’t buy the game on a whim and stop playing a week later. I’d probably call these people more of the “middle layer.” Are they less hardcore than the tourney folks? Sure. But are they not hardcore at all? Well… I wouldn’t say that.
I think that MK has a LOT more of this type of fan than you seem to believe. I really don’t think it’s just a small hardcore audience and then a giant amount of casuals that drop the game in a week or a month. If you disagree though, no problem. 
I do think though that after a year plus, that MKX is on its way out. Casuals make the sales, but even the middle layer and the hardcore can’t keep a game at it’s height in the tourney scene when that scene is so thoroughly dominated by Capcom enthusiasts that would rather saw off their own hands than play MKX.
Yeah, I was around. I was so happy with that game and what it did for the genre. Sure, MK had been trudging along through it’s 3D era (for better or worse), and we were still getting iterations of stuff like Tekken and Soul Calbiur, but SFIV came out when I tend to think the FGC needed it the most. I agree that the original was different from where it ended up in Ultra, so that will be exciting to watch for SFV.
I dunno, I just can’t bring myself to be nearly as excited for SFV as I was for SFIV. The gameplay is solid, but the art style, the character design, I still haven’t found anything in that game that makes me want to latch on and stick with it for a while. Hopefully that’ll change though! The more FGs, the merrier.