I hear this all of the time from authors. There is not other book out there like mine so it will clearly be a big seller! While the thought sounds like it might be on the right track, I think these authors really are missing the point here.
Doing market research is one of the most important things an author can do. This gives an author guidance as to themes, eras, sub-genres and even voice to some extent. In fact, every business does that. It is always important to get on that wave (hopefully early) and ride it out until the next wave hits. But, if there is truly nothing out there like your book, the odds are there is NO MARKET!
I see this a lot on the historical author chats out there on the "Interweb." Authors are writing their novels and spending years on research for time periods that are just not going to sell. There are also authors in these groups that are amazingly angry at the publishers and agents who are not willing to look at their novel. Their books are amazing and their beta-readers (translation their close family) love their book. That's great! But there is not a buyer out there interested in those topics (other than that author, their beta readers and that professor who wrote the one research book on the topic).
And now the authors try to take the argument to a new level. Maybe it will be their book that will start the trend. If you are someone who has been reading my blog for some time, you know I refer to those rare occurrences as anomalies. Those authors are the exception to the rule, and the odds are, there are a lot of other variables they are not discussing with the public that allowed that to happen.
So, does that mean you should not write that novel? No. If your heart says to write it, then do so. Just don't go complaining when no one is interested. You did this because you have a passion. But do remember, book sales are not build simply around the author's passion.