Thierry wonders (I think) whether I should be discussing this subject rather than merely initiating it, so I will respond to his invitation; although I am not about to answer the question, I can at least put forward some points that might be helpful.
The thread was, of course, started in response to a similar one about pianists' incomes.
The "day job" idea begs the question of the amount of time that anyone might reasonably expect a composer to devote to composition which, of course, involves not only writing time but thinking time. Of course some composers think and/or write faster than others, but the act of composition is always comparatively labour-intensive by most people's standards; just imagine, for example, how long it takes to think and notate a mere five seconds of music in a full orchestral score. If a composer has a full-time "day job", it might occupy some 10 hours daily, 5 or more days per week, including travelling and preparation time, so is the amount of time left over for compositional activity sufficient? Even composers have to have some kind of life outside of work!
Another factor is that performers' and composers' incomes are different, in favour of the composer (for once!). A performer simply gets a fee per appearance whereas, when a composer writes a piece, it's usually half fee up front and half on completion in the case of a commissioned work, but it doesn't end there, for the composer will also continue to receive royalties on certain public performances, broadcasts and recordings for the remainder of his/her life; even non-commissioned works still usually attract such royalty payments. In addition, the composer may derive an income stream from sales of scores, whether via a publisher (who takes a cut) or direct by self-publication and distribution (as in my case, which means that more of the proceeds end up in the composer's pocket than when a publisher is involved).
Obviously, composers' incomes vary widely just as performers' do, but I think that fewer people might expect the latter to have a "day job" than might expect the performer to have one.
The "day job" issue reveals another problem in that many such jobs involve teaching; might there not be cause to question the notion that composers teach students to do something from which they themselves will likely be unable to derive a living exclusively? - after all, this would presumably lead to ever-increasing numbers of composition teachers teaching ever-burgeoning numbers of composition students, thereby disadvantaging the future market by flooding it with composition teachers and composers whose "end-users" (the members of the listening public) simply cannot accordingly expand the amount of time and money that they have at their disposal to listen to and pay for ever-increasing quantities of music.
There's a few cents' worth thrown into the ring for discussion; I hope that's useful.
Best,
Alistair