Very true - the first thing anybody wishing to study composition should be aware of is that "being a composer" really isn't a "job" (in the sense of allowing you to make a living) in most cases. The only job a composition diploma will be useful for is if you want to teach composition at an university. While that's certainly a desirable thing for many, such jobs are rare enough and you'll rarely get one right as soon as you're done studying. In all other cases (respectively in the meantime), you will have to find some other way to earn your money.
This doesn't have to be something entirely unrelated to music of course. There -are- certainly ways to earn some money in the music field, from giving instrumental (or theory) lessons, working for a music publisher or as a music critic, assisting music professors, and so on. Some of these will require additional qualifications (i.e. if you want decent pay for giving instrumental lessons you will want an instrumental diploma), whereas others can offer you money on the side, but won't provide you with a full job on their own. -If- you're set on earning your money in the music field, it would definitely be a good idea to also get an instrumental diploma in order to teach, or at least a theory diploma in order to teach theory. (But generally, you still can do these things a bit later as well.)
Personally, I currently earn most of my money by a wide selection of different things, ranging from giving theory lessons to some kids, doing music typesetting jobs for some composers, programming/installing/performing live-electronics for concerts that require them, freelancing as a hornist in some small orchestras from time to time, playing alphorn on weddings and stuff, accompanying singers/instrumentalist on the piano from time to time, etc. On one hand, doing so many things is fun and keeps me from getting bored, on the other hand, it's anything but a job with a fixed monthly income, so in the long term this is much too unsteady to work out very well.
All in all: If you stay open towards several directions you won't starve as a composer. But things may turn out a lot easier if you manage to get some other day job.