Anyways, books are a dying art,
Difficult to get into, yes. Dying, no. E-books, baby. They're all the rage now.
But otherwise, yes, pretty much you can't really make a living off of it unless you happen to be lucky (i.e., come up with the next biggest thing*). That isn't really to say that you shouldn't attempt it. Just that there's a lot you'll want to consider and a lot of side steps no one ever actually thinks about if you actually want to make it. Moreover, it's just safer to keep your day job either way unless you really
are pretty lucky.
Or, well, go into something other than the writing part of the publishing industry. There's lots of awesome jobs out there besides being a writer, after all.
* Note that I said "next biggest thing." If you publish something trendy right now, you run into one of two risks. The first is that if you're lucky and get published at the height of the trend, you'll just be written off as a rip-off book series of something that's better (even if that something is Twilight) or, even worse, the literary equivalent of a sell-out. The second and more likely is that you won't be lucky, and the trendy thing changes before your book can hit the shelves. Think about it. Only a few years before Twilight got popular, everyone was talking about wizards and witches thanks to Harry Potter. A few years ago, everyone started talking about sparkly vampires and werewolves that don't sparkle. While we're still on the vampire craze, there's not really any way to be sure that next year, we're going to be talking about something else.
A published author I once had the pleasure of listening to offered an analogy from her own life. She said that when she tried to publish her first book, it was basically inspired by Bridget Jones's Diary, and that was okay in her view because that was popular at the time. However, by the time she was ready to publish, Bridget Jones was old news. Needless to say, she had to scrap that novel and write one completely from scratch. While she's not at JK Rowling's level, she
does write for a living, and she
is decently popular in the chick lit genre.
In short, you're right about trends on a level. But there's definitely more of a risk to it because you actually have to have a lot of foresight. Publishing houses won't accept anything that's not going to sell well, but they
will accept something that
sounds like it might sell well by starting a brand-new trend. It's like being in the fashion industry. You can't risk accidentally designing something that was more fashionable last season, but you also can't risk designing something that just won't sell at all.
But that's why having a lit agent helps massively. The lit agent would be industry savvy enough to know all this and help point you in the right direction, assuming you're talented enough.