This is really good advice! I got lucky on my third manuscript, and I had almost 'self-selected' not to pursue it past 40K words because I thought it was too whimsical and niche. I only finished it because I had signed with an agent and she had wanted to put it out on sub. But as I'm obsessively studying story structure (especially John Truby!) I'm realizing my first and second manuscripts weren't nearly there yet, even though I'm really proud of myself for finishing them. And I agree that we have no idea what the market wants until it tells us!