Commercial fiction writing today is a hybrid because it is both a money-making endeavor (I refuse to call it a "business") and a means of creative expression. When professional fiction writers get together they tend to talk a lot about things like marketing.
I've been talking with fellow writers a lot recently about changes in the market and how to respond to them.
Quite simply, if you're going to write for money you can't ignore the marketing and commercial aspects of your work. However no matter how commercial you are you've got to complete what you write in order to get paid for it.
If you're not really, truly interested in what you're writing you're unlikely to complete it. If you don't finish it you can't sell it.
This, incidentally, is one of the problems with writing pornography for money. Back in the days when there was still a booming market for "stroke books" nearly every writer considered writing them and not a few tried. After all, the money was almost decent and how hard could it be?
For most writers it was very hard indeed. The problem is that very few of us are than interested in pornography (at least to write) and it quickly tends to become a death market. Writers who tried it found that porn was excruciating boring to write. Some managed to get through one book. Very, very few managed two or more.
The market, fortunately or unfortunately, is no more; killed by the amateur porn on the internet. (Which, be it noted, is often of higher quality than the average stroke book.) However the principle applies. You can see it today in the occasional writer from other genres to decides to try his or her hand at writing romances. The ones who aren't really, truly interested in romances can't stick it out. And because they don't have the interest they produce lousy romances.
Which leads to a critical question for any writer contemplating a book-length project: Will this hold my interest? If the answer is no, don't try it. No matter how commercial the project is and no matter how mercenary you are.
This isn't just a matter of a burst of enthusiasm in the beginning. That's easy and will get you through maybe 10,000 words. It's keeping your interest in what you're doing over the long haul.
Nearly every writer goes through a stage in a book -- usually about 60 percent through -- where the enthusiasm has evaporated and the thing becomes a slog. It can be a real death march, worse than any programming marathon. You've got to have the internal resources to get through that period and if the story and the characters don't grab you you're not going to make it..