We’re disagreeing mostly on a point of emphasis.
Popular music is accessible by definition. Classical music was broadly popular deep into the 20th century, and adapted to popular tastes in the form of operettas which had huge appeal on Broadway and in the movies into the 1930s and 1940s, and in standards for a generation after. Only a disdain for popular appeal moved classical almost completely into a nostalgic sideline for dead composers. Similarly, jazz swing was mainstream popular music until bebop came along, and that still had a solid audience until it deliberately became almost flamboyantly inaccessible. Jazz insiders still haven’t forgiven jazz-rock fusion for being popular by using the accessibility of rock to hook its audience.
There has always been inaccessible music in the vast and now splintered conglomeration that I’ll refer to as rock as well, whether influential or not. (Zappa being more influential than, say, Captain Beefheart.) Accessible music has always dominated, though, and artists, producers and record companies have deliberately taken inaccessible rock and moved it toward the mainstream, from r&b in the 50s to hip-hop today.
Any broadly accessible artist will stand out even in the fractured future. Outkast cut through lots of genres and became so ubiquitous that people got sick of them. Music this popular will reach out from its core audience. I admit I never even heard "Hey Ya’ until after someone here on the Dope complained about its being overplayed. Once I found it, though, I started hearing it everywhere. (Or maybe I didn’t catch it as late as all that. Early adopters, in the industry phrase, lose interest the earliest too.)
From today’s vantage it looks like no future Beatles could conquer the music universe. But nobody expected the Beatles either. They could not have been imaged until they arrived. Somebody in some form of music will be as talented as the Beatles and will move the entire music world for the simple reason that there will be money to be made by copying them.
There’ll always be an underground too. But popular music arises from the underground. Nirvana’s first album was underground. So too The Clash, and the B-52s, and disco and hip-hop and for that matter the Velvet Underground, of whom it was said facetiously that they sold only a thousand albums but every single buyer formed a band. If people want to turn indie rock into acid jazz and keep it all to themselves, then it will become as dead as classical. But something else will take its place and rise up.
