A Mozart puzzle...

If I don't practice one day, I know it. Two days, the critics know it. Three days, the public knows it.
JASCHA HEIFETZ
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In his book Mozart: A Life, Maynard Solomon gives the following description of Mozart's typical daily routine:

During his early years in Vienna, Mozart would customarily arise at six o'clock, be at his desk by seven, and compose until nine or ten, when he would make the rounds of his pupils, giving lessons until one o'clock.  "Then I lunch," he reported to his sister...  Returning to his room after several hours of social visits, he would again compose ... "I often go on writing until one - and am up again at six." ... With variations, that was Mozart's daily routine as he described it in his letters home... [On some] days the only time he had for composing was in the evenings, "and of that I can never be sure, as I am often asked to perform at concerts." (p.309).

With Mozart's time divided between composing, teaching, socializing, and frequent concert performances, the above description of his daily routine suggests that Mozart had no time to practice at all, or at least that he did not practice regularly enough to warrant mentioning practice among his daily activities.  This I find very hard to believe, but since I have no compelling evidence to the contrary, the best I can do is offer a few rather inclusive speculations on this matter.

Modernism as an attitude problem

Mr. Carter never has made concessions to his listeners. ... It will take many hearings for the relationships in the score to assert themselves.
HAROLD C. SCHONBERG [1]
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So there you have it: An innovative composer's pursuit of his artistic vision is an attitude problem.  The composer is described as one who refuses to "make concessions to his listeners", who is "uncompromising", who expects his listeners do heavy mental work involved in keeping track of a bewilderingly rapid succession of seemingly unrelated sound events.  Put in a euphemism-free way, the composer is an arrogant motherfucker who pursues his aesthetic ideals at the expense of his listeners' desire for pleasantly comfortable aural experiences after a long and busy day at the office (or at the country club).