Depth starts hurting at the point where thoroughness stops serving the answer and starts hiding it. Up to that point more depth helps: it covers the sub-questions, builds the case, earns the reader’s trust. The turn comes when the depth gets in front of the thing the searcher came for, when the direct answer sits three sections down, behind preamble and definitions and caveats they have to read past to reach it. The page may be more complete than any rival, but completeness the reader can’t get to quickly reads as friction, not value.
From there the consequence runs through behavior. A searcher who clicks, scans, and doesn’t see the answer fast tends to assume it isn’t there and returns to the results, often to try a competitor. That return is a reasonably clear sign the page didn’t satisfy the click. The link to ranking is real but indirect, and worth stating honestly: Google has stated it doesn’t use that bounce-back directly as a ranking signal, because people go back to search for many reasons. But over time the system tends to surface the result searchers prefer, so a page that consistently makes readers dig and leave can lose ground to one that answers sooner. Treat the behavior-to-ranking link as observed pressure, not a published rule.
The harm, then, isn’t the depth itself. It’s depth placed in front of the answer instead of behind it. A thorough page and a fast answer aren’t in conflict; the order is what decides whether the depth helps or costs you.
Find where your deep page hides its main answer, and move that answer up so the reader gets it first, then let the depth follow for those who want it, before the burial costs you the click.