Hi Schubert - we do mostly agree. Yes, Mozart and Schubert wrote a few compositions in a very short amount of time. This has much to do with the fact that every musician back then had and used much more improvisational skills than most of us do today, with the exception of some jazz musicians and baroque/classical keyboard players. However, I can assure you that neither Mozart or Schubert wrote an entire symphony "in his head" and then wrote it down afterwards. A theme, yes, a short lieder, yes, a simple dance movement for piano such as a minuet, yes, a large multi-movement composition, no. In other words, anything they wrote out that quickly was essentially an improvisation. There are some excellent books out there that speak of Mozart's and Schubert's compositional processes. Both men would work incredibly long hours, with very detrimental effects on their health (that part of the movie Amadeus is very accurate). They were two of the hardest working composers in history, especially when their too too brief lifespans are considered, as Frogman mentioned. I have said in these forums before that I consider the death of Mozart to be the most tragic early death in the history of all the arts, not just music.
You mentioned Bruckner, also a favorite of mine. He was very famous for his improvisations at the organ, none of which he wrote down afterwards, unfortunately for posterity. According to many contemporary accounts, some of them were better than the solo organ works that he did write down. The same goes for J. S. Bach, though of course in his case, everything he wrote down was pretty much a masterwork. Bruckner took much more time to develop as a composer than the others you mentioned. What he became a master of was the extension of long forms, which of course could not be improvised, hence the longer developmental period in his case. He was another workaholic, like pretty much all truly great artists.
You mentioned Bruckner, also a favorite of mine. He was very famous for his improvisations at the organ, none of which he wrote down afterwards, unfortunately for posterity. According to many contemporary accounts, some of them were better than the solo organ works that he did write down. The same goes for J. S. Bach, though of course in his case, everything he wrote down was pretty much a masterwork. Bruckner took much more time to develop as a composer than the others you mentioned. What he became a master of was the extension of long forms, which of course could not be improvised, hence the longer developmental period in his case. He was another workaholic, like pretty much all truly great artists.