I agree with Flash and Mosh, although perhaps in a different way.
Call it prog or not, call it complex or not, I've noticed that adventurous, long songs with many (unexpected) changes are not that popular out here. With some accessible bands, I find that odd (e.g. Iced Earth, Helloween) because this is a Maiden forum (these bands were influenced by Maiden; besides stuff from other bands, they took trademark Maiden ingredients and sauced these with their own sound and made an own identity) and long songs as I described are Maiden trademarks. For some reason people tolerate it from this band but hardly from others. Opeth, Dream Theater, Symphony X, we can name them, but I don't think that there are more than a handful of people who are into these bands. Rush and Floyd are probably -besides Maiden- the only bands with long songs that have appreciation of some sort.
However, I do not think that this King Crimson song or perhaps even the band King Crimson sounds mainstream. Also not when you look at prog rock only. Perhaps this song sounds mainstream for some King Crimson fans (I am not one of them; I really like some more accessible songs on the debut but for the rest I think that lots of their music sound a bit over the top and/or too repetitive), but in that case In the Court of the Crimson King and Epitaph sound like number one hits in the pop charts.