Is prog still an underground genre?

Underground? No, it's rather a "niche", just like goth rock, abstract hip-hop or cumbia outside of South America.

The fact that some bands as Porcupine Tree or Mars Volta (not forgetting about Muse or Radiohead) are somewhat big sellers don't mean that a large audience cares about the classics (except for Genesis or Pink Floyd) or the Neo Prog (except Marillion, which still can fill some middlesized venues in Paris - not Lyon, not Marseille) or even newer bands.

Oh, and let me tell you: among most of musical circles I could make my way through (Jazz, Alt/Indie Pop Rock, so-called experimental rock), Prog is NOT seen as "cool", "fresh" or even acceptable.

I even heard a friend of mine calling the music of Flying Luttenbachers as "fascist" (I'm still trying to understand...) or my brother warning me that "there's no turning back after starting listening to Robert Wyatt"!

RYM is much larger. Just look at the numbers. 

PA's #1 is Close To The Edge with 4104 ratings.

RYM has 13,286 ratings for it. RYM is much more popular and if you scan the entire charts and count the number of prog albums, it's quite large.

Prog is mainstream. Everyone has heard of the biggest bands within it. Have you ever met one person who hasn't heard of Crimson, Floyd, Tull, Yes, etc? I haven't.

I rattled off numerous prog bands that had gold and platinum albums in the 70s (and everything Genesis released went gold in the States from Selling England by the Pound onward, and their first platinum was And Then There Were Three -- it's not like they were "underground" in Britain, France or the States from '74 onward), and I also indicated several non-prog bands like The Who and Zeppelin suddenly started coming out with very prog songs. Even Alice Cooper and Black Sabbath got proggy for a minute.

Prog was an actual "thing" at the time, and the difficulty of the songs, requiring elite instrumental mastery, lengthy compositions and at least a passing knowledge of classical or jazz music led to the inevitable backlash of the devolved and simplistic Punk movement by the mid 70s (helped along by the music industry and NY and London t**t critics). But it's not like Prog suddenly disappeared. I went to numerous sold out prog concerts at major venues in the late 70s (Floyd, Tull, Genesis and Yes).

P.S. I guess what I am saying is that "prog" was an acceptable part of the average rock fans diet in the 70s. Experimentation, as evidenced by the rise and popularity of jazz-fusion or the Allman Brothers and Grateful Dead playing 10 or 20 minute long improvisations was part of the listening process. 

It's the reason why a Yes album with 3 songs goes multi-platinum, or Tull's Thick as a Brick and A Passion Play can go to #1 in the States, or that Queen can release the 6 minute long "Bohemian Rhapsody" and sell a million singles, or Led Zeppelin had fans drooling over the 7 minute long "No Quarter" or the 8:37 long "Kashmir". Hell, even "Stairway to Heaven" was over 8 minutes long and you'd hear it on the radio nearly every day. I remember even a 12 minute long song like Traffic's "Low Spark of High Heeled Boys" got enough airplay to be voted to a Top 100 songs list by listeners for several years on FM Stations in Detroit.

Just look at Rate Your Music's top 5000 list. Just in the top 100 are 16 albums that are considered prog on PA. This is a site that rates ALL music. True the classics are still the most popular but they are more popular than EVER! So NOOOOOOO prog is mainstream.

In that time period you also had major rock bands who weren't necessarily into "prog" by definition playing decidedly prog compositions: Led Zeppelin, The Who, Queen and Santana, as well as jazz taking a decided rock sound with the fusion movement led by Mahavishnu Orchestra, Return to Forever, Jeff Beck, Pat Metheny and Billy Cobham.

Even second-tier bands like Camel, Caravan, PFM, Renaissance, Banco Del Mutuo Soccorso, Strawbs, Nektar, Amon Duul II, Van der Graaf Generator and a long list of Canterbury collectives had decent careers during that era. Not to mention Can, Tangerine Dream, Gong, Faust, Kraftwerk and the Krautrock crowd.

If anything, the golden age passed and the genre has regressed into the back alley. Marillion had platinum albums in Britain during the 80s, Yes and Rush continued to sell well, Tull had a couple gold albums in the 80s, and a couple Dream Theater and Mars Volta albums went gold in the 1990s and early 2000s. Steven Wilson isn't really even a big selling performer in relation to the folks he emulated in the 1970s.

It's a fringe niche now, really. Rock in general is fading from the market.