IDEALLY, everyone who works doing anything, including writing, will love what they do! Of course we know that's not true. In the ideal world we love what we do, we are knowledgeable in every facet of what we do, we are among the most competent of all in what we do....we are just great! And when it comes to financial success as a writer, the book written needs to be in demand, have a large audience, and people (i.e., publishers) who will buy same and pay money for it.
So, unto itself is love of what we do enough to guarantee financial success --- absolutely not, although it's a lofty goal we should all aspire to.
I love writing and have been doing so for many years with a degree of financial success. However, my first love was basketball - I wanted to become a professional basketball player, and I can assure you that I worked very hard to try and make that become a reality. I got up to semi pro ball but that's about as far as my ability would take me. The dream and the hope of making "big money" burst. I then turned full time to writing and publishing. But the point remains, I loved basketball, was probably better that 99% of the population, I worked real hard at it, but in the end I simply wasn't good enough. Just another nobody, so to speak. So to say financial success will follow a love of writing or anything else is, I believe, far too shallow a statement.