It's not popular in Russia, Have you ever been there? It's about as popular there as rugby is in the US. Very much a niche sport with little following. Wrestling trounces MMA in popularity, while the reverse is true here in the us. The point is, your logic ignores sample size, one of the most important determinants of whether an argument has statistical value. Yours doesn't.
Japan was good at wrestling a long time ago. It has since become a very average country with occasional flashes of its past brilliance (the globally dominant women's program, which generates a negligible amount of MMA talent, excepted). And what do you think the background of Japan's MMA stars often includes? A lot of them wrestled freestyle as part of their training, supporting the point that freestyle is a good base for MMA and might be as good as folkstyle.
Again, I am not arguing against folkstyle being the best wrestling style for MMA. What I'm saying is that your comments are not proof that that's the case and are in some cases just not true. There's no statistically valid evidence to point to folkstyle being the dominant style. Not only is the folkstyle sample grossly overweight folkstyle wrestlers versus other styles, it is also the preferred style of the USA, where MMA has the widest opportunities for participation and the largest sample size of participants in the world. Moreover, for all the guys who did well in MMA, there are many more who failed. Some even had top 1% folkstyle pedigrees (e.g. Alan Fried, Cary Kolat, Royce Alger, etc.). So on a % basis, folkstyle may not even stack up favorably to other styles, I don't know.
Just curious -- How popular is wrestling in the majority of the Russian population? It looks to me like most of them couldn't care less, but a relatively small minority of them live and breath it.