I think our disagreement is simply a matter of degree; I don't doubt your knowledge as someone who grew up in Cuba! Of course rumba is not nearly as popular as Regueton or pop -- or even salsa, although salsa isn't that popular these days anymore (and certainly it's not great listening music, absent dance). I do think it deserves to be considered mainstream, though, in that most Cubans have at least basic familiarity with what rumba is -- they'll know the Munequitos de Matanzas if no one else, and they'll have seen quite a lot of rumba danced, even if it's not their cup of tea. And wide swathes of society dance enough rumba to convey what dancers call the 'intencion' of the dance, if not necessarily technique.
I'm sure this does depend on location. One of my best friends grew up in San Luis, a small town outside Santiago -- far from a tourist spot -- and danced a lot of rumba growing up. Another friend is from Marianao and grew up playing the music, though not dancing. Some friends in Centro Habana danced plenty. I suspect that growing up in, say, Vedado would be very different, though.
Of course, you are also right that rumba on the street is usually a far, far cry from the rumba danced by the pros -- but then, the average Cuban's casino is also pretty weak. That doesn't mean that the rumba (or casino) danced in the street isn't valid for some reason (in fact, sometimes I tend to think it is more 'valid' than the sophisticated beautiful pro rumba).
Contrast that with dances like pilon, mozambique, danzon. Correct me if I'm wrong -- my impression is that most people will have heard of these, but the level of knowledge among the general public is much lower than for rumba. Even son today I'm not sure I would call mainstream -- the music yes, but the dance seems to have lost its popular appeal.
But again, I think we are simply arguing over definitions. Naturally, after 18 years in Cuba you know more about the situation on the ground.