This is a total red herring. If Adele/Taylor Swift/Lady Gaga was to release a 24-minute song on one of their albums which was larded up with extra musicians and contained instrumental solos and time changes, it would go to No. 1 simply by virtue of who released it. And it could be the most god-awful thing ever made (not to suggest that their music in general is god-awful; I'm just using as an example the biggest sellers in popular music today to make a point about popularity and commercial success).
Bands who do this sort of thing for a living? Not so fashionable these days. So, *nobody* doing that on the regular has any chance to have a hit album. Epic length songs being made by progressive rock bands had a brief sliver of commercial viability and then that time passed (a very long time ago). But even then, commercial success had a TON to do with being on a major label and the quality of your promotion. In other words, the success of those products in the marketplace had quite a bit to do with the quality of marketing and distribution - totally non-artistic factors.
I've heard many "progressive" musicians opine about how difficult it is to write a good 3.5 minute pop song, so I'll defer to them. It may very well be harder to do that.
But if "how hard is it" is the main factor for you, I'd suggest selling off your progressive rock albums and focusing almost exclusively on classical music, as that arena is substantially more difficult than progressive rock "epics."
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