Down to brass tacks: Is anyone in your list still relevant? Close call. Bowie was irrelevant by 1980, so he put out a pop album and cruised on royalties. Gabriel was irrelevant by 1983; he made "So" (which was short for "So Lame I Didn't Bother with a Title") and cruised on the royalties. Plant was irrelevant in 1969, when Zep xeroxed the Jeff Beck Group (who were already harder, louder, and more accomplished musically, plus not dorks); he and Page became re-issue kings and cruised on royalties. Bruce was irrelevant in 1982. "Nebraska" and "The River" (both great records) confused his core audience, so he made "Born in the USA" and cruised on the royalties. (He did later return to his darker, slower stuff on "Tom Joad," but the Jersey meatheads still weren't buying. Kudos to Bruce for trying again, but he's still irrelevant.) As for Costello. He made a bunch of trashy, fun shit in the 70s, tried to get pop-sophisticated in the 80s, tried to get respect in the 90s, then made some more trashy, fun shit on "When I Was Cruel." Have you heard "Tear Off Your Own Head" off that particular record? It's got a great riff, a great chorus, a great beat, and it sounds wicked good cranked to eleven.
Re your second point (no more rock, only subgenres), it's bracing to consider rock's history. Very few artists in the past 30 years have done anything the Beatles didn't do first or better. Like REM? Listen to "We Can Work It Out." Like Soundgarden? Listen to "Helter Skelter." (About the only thing the Beatles didn't do well was country, which always came out sinding a little too Wild West rather than Nashville. On the other hand, they started their career doing Buddy Holly covers, so they knew the form.) I think of the Beatles as the great vertical axis in rock, and nearly everyone else as lines on the horizontal axis, emerging from some point on the vertical.