Setting the extraordinary technical difficulty of the music of Domenico Scarlatti and Bach against the keyboard music of the later part of the century, one might think that keyboard technique had deteriorated; in fact, the market for piano music had expanded.

It was Beethoven who felt that the desires of the amateur -- or even of the average professional -- were not worth attending to except when he wrote an easy piece to make a little extra money. (Even then, his idea of an easy piece -- for example, the first movement of opus 79 -- was likely to deter the average amateur, just as Mozart composed one of his hardest works -- the Sonata in D Major K. 576 -- under the mistaken impression that he was producing something that could be negotiated by a beginner or an amateur.)

Charles Rosen, Piano Notes: The World of the Pianist