"If there is such a thing as the march of history in music, it is certainly not a uniform movement; on the contrary, the more closely it is examined the more it is seen to be varied, tumultuous, and contradictory. There are always individuals and groups who are out of step: some drag back, some press forward ahead of the crowd, some move in other than what seems to be the prevailing direction. Moreover, each individual, if his work is of any significance at all, has something to say that is unique and that cannot be adequately subsumed under any general description of the period."
Donald Jay Grout in 'A History of Western Music', WW Norton & Co, New York / London 1980, page 401.
In the light of this common-sense observation, any prescriptive idea about how contemporary music should sound, can be considered uninformed and totalitarian.
But Grout still looks upon history as if on a time line, with a 'forward', and thus also a 'backward', so that the composer who is presenting something original and unusual, is seen as producing something 'new' on the time line, instead of something original in the context of his historic environment. So, Debussy is 'avantgarde' in the context of his time, while it would be better to see his work as something very original. The confusion of historical placing and artistic value and meaning has created the absurdity that works are judged artistically according to their being 'new' on the historic time line, instead of their intrinsic value.
For instance, Stravinsky's Octet is often hailed as a groundbreaking work leading into his neoclassical period, giving it an importance which is partly the result of its historic placing. But is it a good work on its own accord? Stravinsky has expressed himself very positively about Satie, because he gave him a couple of hints for his neoclassical aesthetic (although he could not bear Satie's regularity), and compared Ravel negatively to Satie by labelling him (Ravel) as conventional. What a blunder.... Ravel is an infinitely more gifted artist than Satie, whose limitations shaped his originality. The artistic qualities and personal character of Ravel's work should, by now, not be subject to doubt: almost all of his works are original master pieces. But Satie seemed to explore an aesthetic which became trendy in the Parisian twenties, so his position on the time line gave him a reputation not altogether balanced by the musical substance of his works.
The same problem with Wagner: 'Tristan' is considered the non plus ultra of premodernism, leading towards atonality, while 'Meistersinger' and 'Parsifal' are often suggested as being 'less important' in comparison because their idiom does not match Tristan's extraordinary chromaticism. But 'Meistersinger' could also be considered the result of a restorative impulse, with its neobaroque elements, and 'pointing towards' the postmodern situation as predicted by Leonard B. Meyer in his 'Music, the Arts, and Ideas' where he describes the future of 20C music as probably resulting in a situation of a 'fluctuating stasis', a bold idea when the book was written: 1967, when modernism raged through music life. (University of Chicago Press, Chicago / London). In a fluctuating stasis, aesthetic ideas bubble-up and take-on some prominence for a while, only to withdraw into the background of public awareness again to make place for other ideas, and all of them circulate within a cultural field which can be best described, metaphorically, as a space in which historical possibilities freely float to be reworked and/or developed free from historical determination.
The German composer Bernd Alois Zimmermann (1918-1970) came-up with the idea of a 'Kugelform der Zeit', a metaphor where all different times exist together in one mental space, without a 'time line'. He tried to realize this in his controversial opera 'Die Soldaten', which is a twelve-tone 'Totaltheater' where indeed things happen simultaneously - a rather literal interpretation of his idea. But the timelessness of his Kugelform idea touches on an a-historical vision which may only become better understood in the 21st century: nowadays, with so many sources of information being accessible, many different approaches of 'contemporary music' are being written and presented, in which what happened in history is repeated, developed, interpreted, reworked, restored, - including postwar modernism (sonic art), surprisingly. But it is a truly good prospect if pluralism is more widely accepted and works judged according to their artistic merits instead of their style or historic references.