State Farm denied my water damage claim saying it was 'gradual damage' - is there any way to fight this?

My basement flooded after a pipe in the wall slowly leaked over time. Didn't notice until the drywall was already destroyed. Filed with State Farm and they came back with a denial citing "gradual damage" as the reason - apparently it wasn't sudden enough to be covered.

I understand there's a distinction between sudden damage and gradual, but honestly how was I supposed to know a pipe inside a wall was leaking? There's no way to see that. My adjuster was pretty dismissive about the whole thing and basically said "you should have had it inspected."

Has anyone successfully appealed a gradual damage denial? The repair estimate is about $18,000 and I really can't eat that cost. Any advice on what documentation helps or whether a public adjuster is worth hiring at this point?