What I’ve seen in adult fantasy over the last few years is that big advance / publisher support isn’t enough to guarantee success. The book still has to be good.
I’m not going to mention the book’s name but there was a fantasy author whose first book was released about two years ago with a ton of hype - hundreds of Goodreads reviews from a dedicated ARC campaign before the book was released, lots of other authors and bloggers swooning over the book, a groundswell of excitement on r/fantasy and in fantasy Facebook groups and blogs, blog tour / signings / speaking engagements, etc. She was given a six-figure advance and anointed as a new queen of the genre. I was caught up in the hype and bought the book on release, when the ARC ratings on Goodreads had it at 4.4 / 5, or something like that. There’s no doubt in my mind that this author getting a 6-figure advance was a reason why the publishing house went all out in promotion. So I read the book. It was awful, one of the worst books I’ve finished in years. I was flabbergasted how my opinion could diverge so greatly from what I was seeing on blogs and Goodreads. Well, fast forward two years. Despite the huge promotion campaign the book has about 1k Goodreads ratings, which is not a lot 2 years in for a book that was intensely promoted. The Goodreads average has gone from 4.4 to 3.5.
Now contrast that with Kings of the Wyld. It was released about the same time. Was bought for a tiny amount, and had no pre-publication buzz. The author is still working as a waiter because the advance was minimal - I think around 10k. Got released straight to paperback. And yet . . . buzz started to build. People started raving about the book - not the ARC reviewers who gush over whatever Tor or Orbit sends them, but real readers. I read the book as this groundswell reached my ears. It was amazing, maybe my favorite book of 2017. Now it has a 4.35 average on Goodreads and over 13k ratings.
So, basically, my point is that publishing companies are less successful about cramming books they think ‘should’ be bestsellers down our throats than they once were. The ease of sharing opinions online has I think democratized success - maybe once a coordinated ARC campaign could fool everyone into thinking a book was great and that their opinions were wrong, but, nowadays, if the emperor has no clothes it’s too easy for the masses to get on their computers and point that out.
I’m seeing a lot of hyped trad books stumble because the publisher is wrong about their appeal, and they are no longer the ultimate arbiters of taste because they’ve lost control of the levers of the industry that used to be solely theirs.
So I do have confidence that a book without a large advance can make it, maybe more often now than in the past. Being more connected through the internet makes it easier for great things to go viral, and, likewise, for sub-par stuff to be recognized for what it is, despite what publishers want the readers to believe.
Summation: Advances, I think, do probably matter in regards to how much promotion effort is given to books. But in the end a book rises and falls on its own merits, particularly in the current age, when we are not reliant on a few majordomos through a few channels telling us what is good.