You see, you could argue the opposite pretty easily: the early Baroque music was crude compared to the refined counterpoint of the late 16th century. Early Classical music was crude compared to the heights of the High Baroque (just compare the music of J. S. Bach to that of his sons). Cubist painting crude compared to the best examples of French Impressionism and so on. It's complicated.
Yet there is some kind of progress in the arts because history does not, in fact, repeat itself, despite the frequent claim to the contrary. But progress in the arts is by fits and starts and the measurement of it is haphazard. There are a near-infinite number of variables influencing artistic creation: the aesthetic needs and tastes of the society, the aesthetic goals of the artist, the economics of the arts, the nature of the materials available, the recent history of the art form and on and on. Underlying it all is the way creativity in the arts works.
Of the Big Five psychological traits, the one that is most responsible for creativity is Openness. The artist must be open, of course, to new ideas. A useful metaphor might be that ideas or themes are like neutrinos, invisibly sleeting through all of us all the time, but only a few of us are aware of them and even fewer are able to make use of them in artistic creation. Musicians and artists often speak of themes just "coming to them" or of stumbling across them randomly.
The corollary to this is that, as a Zen master said, the cup must be empty before you can fill it. He was referring to learning about Zen, but it applies in many situations. The act of creation is often preceded by the act of clearing away. Before Haydn and others could invent the crisp clarity and balance of the Classical Style they had to clear away all the thick textures and complex harmonies of the High Baroque, replacing them with opera buffa inspired jostling rhythms. Before Steve Reich could create his monolithic rhythmic structures he had to clear away virtually everything of 20th century modernism: no more dissonance, intricate pitch structures, fragmentary rhythms and so on. They were all replaced by little more than pulse at first.
This even applies to pop music. The punk artists of the mid-70s sneeringly threw away all the pretensions and complexities of "art rock" in favor of the simplest and crudest textures they could find. Similarly, performers of rap and hip-hop got rid of harmony and melody in favor of the rhetoric of speech, sampling and drum tracks.
This is not the only way the arts progress, by excision and the upsetting of priorities (rhythm over harmony, speech over singing, pulse over melody, etc.); sometimes a new era in the arts consists of addition and a change in perspective. The Romantic Era in music added harmonic and melodic complexity to the Classical Style without changing the fundamental bases. The best example of that is the music of Franz Schubert, undeniably Romantic in its sensibility, but based very firmly on the Classical structures.
That's probably enough musing for today, let's have a musical example. Well, two actually. First, a Haydn piano sonata, which is a good example of the pure Classical style, second a Schubert piano sonata which, despite the great differences (not least in length), is still based largely on the Classical foundation.
Haydn: Piano Sonata nº 59 in E flat:
Schubert: Piano Sonata No 18 in G major, D 894