Quads are now mostly a niche for those who like/want military clone builds. And that niche will probably stick around for as long as the military finds value in quads (though even .mil is looking to move to M-Lok for some units).
The broader market has moved away for reasons already mentioned - weight and comfort, and now there is a suitable replacement standard for 1913/picatinny for the sides and bottom of the handguard (when non-quad tubular handguards first came out, e.g., Troy, Midwest Industries, etc., there was no standard accessory attachment mechanism, which limited market penetration), namely M-Lok.