To a certain degree postminimalism + new electronica + noise + sound art + free improvisation + EAI + some "new complexity" composers should all be classified together as a new genre, distinct from "classical" and "popular" musics although deriving from both. If there is a 21st century mainstream it's embodied in a focus on performance (or rather, a return to a focus on performance, after 200-300 years of the focus being on an abstract process called "composition") and specifically those aspects of performance that can't be easily translated into recorded sound. The time period in which recordings were central is a blip in history; a gap of fifty years between the invention of magnetic tape and the invention of the internet. Now that recordings are digital and digital media are free, pop music, which used the recording as the embodiment of the work, is forced to evolve. This is a good thing. The concept of the work, whether a score or a recording, is problematic, and is part of cultural narratives we are beginning to abandon as a society.
Regardless. I agree that much of the work of the past 40-50 years will come to be regarded as, if not bad, at least reactionary (which is often the same thing). Many musicians shied away from the tough questions that had been posed in the fifties and 'sixties in the aftermath of the worst disaster in human history. Pierre Boulez obsessively reworked his early, epoch-making pieces, each reimagining less striking than the last. The Germans turned out infinite imitations of Lachenmann and Stockhausen that addressed the surface rather than the ideas behind their music. The Americans churned out bland serialist music for a few years before returning to their prewar tendencies of churning out bland neoromantic music. (Of course, they were well out of the war.) Minimalism was a poor attempt to achieve some of the intensity and power of pop music, which itself was repeatedly sanitized and neutered for the enjoyment of middle-class white audiences. Here we are in the twenty-first century now. The classical audience is older, richer and more fascist. They demand composers write a certain kind of music, and composers (who largely are the children of that same audience) are happy to oblige. As for pop music, it is developing a "canon" of essential recordings, to which younger artists are denied entry. When the recording companies die it will too, as tonal music died with the social order that supported it.
"Style" is a red herring. Some 20th/21st century composers who had a real impact, on classical, pop and whatever will take their place—and on the wider society—include Strauss, Stravinsky, Webern, Varèse, Cage, Stockhausen, Ligeti, Reich, Cardew, Radulescu, Feldman, Penderecki, the GRM, L'itineraire, — and moving outside the strict idea of "composition", Young, Riley, Zorn, Fluxus, Bang on a Can, and more i can't think of right now.* You probably don't like any of those people because you think music ought to be like it was in the nineteenth century, and that's fine. Not everyone thinks the world is a better place with women wearing trousers, nonwhites being allowed to rule themselves and gays being alive. To claim none of those people/groups was remotely influential however is rather blind.
* also Rachmaninov and Puccini and Korngold, but it's kind of cheating to include them >.>