Had Brahms premiered a sonata that sounded like it belonged with powdered wigs, he would have come off stage with bits of rotten tomato stuck to his face. Mozart could write like Mozart, and Brahms could write like Brahms, and it's possible Mozart could have even written like Brahms, but Brahms could never write like Mozart. Nobody would listen.
Whatever Classical music is, it is progressive. It leans forward constantly. Any backward glance must be through this progressive lens, like Brahms glancing back at Haydn in theme and variation or like Grieg glancing back at the Baroque suite in the Holberg or like Vaughn Williams glancing back at Medieval fauxbourdon in O Vos Omnes. All of these works remain products of their time, however, and cannot be understand as anything other than the specific cultural output of the culture in which they were composed. Which is why Brahms would need to wear a powdered wig to write like Mozart.
This is a sine qua non of Classical music. Without this progressive element, it simply falls apart. Which is why it is impossible to consider the body of music currently being contributed into the Classical canon as valid Classical music. It isn't music that looks forward with any new theoretical lens. It isn't discovering chromaticism like Beethoven. It isn't dispensing with a consistent tonal center like Wagner. It isn't edging on modality like Ravel. It isn't using inordinate dissonance like Messiaen. It isn't being atonal like Schoenburg. It's pretending to sound like Rachmaninoff or Prokofiev or even Webern, but that's fallacious - Classical music must be a unique output of the culture in which it is placed, and our culture is distinctly different from the culture present in the 1950s or the 1920s or the 1880s.
Classical music is dead like Latin is dead. There's nothing new being added to it. You can take some roots and some prefixes and some suffixes and bring them together to have a slightly modified denotation, as might be necessary if you were working for the Vatican and wanted to put together a Papal encyclical. But Latin is still dead. It's not being changed or morphed inside the tongues and minds of Latinists. It's analyzed, studied, and prodded, but not spoken and certainly not changing.
But Classical music is only even possible in a society that values secularism and ars gratia artis. That's rendered moot by a society overrun with a strange mix of Kantianism and utilitarianism, which are, coincidentally, two of the monumental products of secularism. Classical music is oriented around the stage, which is itself only supportable in a society that values schola or leisure the way Rawls might value justice. Leisure, schola, and hence the stage are implausible constructs in a modern and post-modern society. Classical music is no longer possible since it would be a product of a society that cannot philosophically support the concept of classical.
There is a forgotten kind of High music, one whose teleology is lost on our music historians, analysts, and musicologists. It's High music that values leisure, necessitates schola, but rejects the stage, ars gratia artis, and other secular constructs. Music that centers around the altar, ars gratia Dei, etc. Because this is music, it is a product of a certain culture, but in this case, it is far more potent, efficacious, and sacramental in its makeup because it is the cultus, the center of worship. A culture that adopts an attitude of worship will see its culture molded and formed by the music it sings in its worship, since, at the center of every culture is cultus, your manner of worship.
What would our culture look like if we sang OCP? What would it look like if we sang Arvo Pärt?