That sounds fairly ridiculous, do you have a source? The only reason I know that the crossover won't happen are producers/creators of SF don't want someone like Chun-Li getting her head ripped off.
By who? Tournament players? Because tournament players are dedicated (and small) but to say older MK isn't being played when there's a
UMK3 focused tournament in the works with a $1,000 pot bonus is incorrect. If you mean by casuals then it's hard to gauge how much people play those games.
Agree and disagree. They're only hard if you're just getting into it and aren't used to the juggle mechanic/strings. To someone that has played a recent NRS game before it's fairly easy to get combos down in a couple hours. I just picked up D'vorah, who has run cancel combos, and I was doing her tougher ones after 30 minutes or so.
Edit:
Then how would you gauge popularity?