Anne Lyle
Fantastical historian
What if health gets in the way, for example, and you can't sit for hours at a time to write, or you can't write every day or even every week? What if you try really hard but you *can't* write a book a year, or you know there's no way you can, realistically? I feel it would seriously affect the quality of my work; I don't know about others. And unless you're a household name, I doubt publishers would want a book every two years...
Most publishers are willing to be flexible provided you behave professionally and keep them in the loop - I know Douglas Hulick is running about a year behind on his book, and of course Scott Lynch hasn't had anything new out for what, five years? owing to depression. Of course the better your first book does (c.f. Lynch) the more patient your publishers are likely to be.
If you can't keep up a book-a-year pace, it's not the end of the world. You could write two books and sell them both when you're done, or get a one-book deal with an option on a sequel (which you've already started during the contract process), or self-publish at your own pace - there's no one-size-fits-all solution any more...