A plateau is the ceiling when the page has maxed out its intent-fit and the domain and competition will not yield more without a different asset, and it is fixable when a real gap still remains to close. The deciding question is whether there is genuine room to improve the page on a dimension that matters, or whether you have done everything the page itself can do and are now bumping against constraints that a better page cannot overcome. Both outcomes are legitimate, and accepting a ceiling is a real decision rather than a failure.
The pivot is gap-remains versus maxed-against-competition. A gap remains, and the plateau is fixable, when you can still improve intent-fit, depth, format, or authority in a way the top results have and your page lacks. The plateau is the ceiling when your page already matches the intent as well as anything could, the format is right, and the only thing separating you from the top is domain strength or competitor authority that this page alone cannot manufacture. At that point further optimization returns nothing, because the limit is not in the page.
This is why neither pole holds. “Keep optimizing forever” burns effort against a wall once the gap is closed, polishing a page that has nothing left to gain. “Give up on plateaued pages” abandons pages that still have an obvious gap a competitor exploited, leaving easy ranking on the table. The right move is to judge which situation you are in before deciding whether to keep investing, because the same plateau means opposite things depending on whether a gap remains.
Before you put more into a plateaued page, audit it against the results above it for any real gap in intent, depth, format, or authority. If you find one, it is fixable, so close it. If you find none and the only difference is domain-level competition, accept the ceiling and redirect that effort to a page or asset where a gap still exists. Decide by whether a real gap remains, and treat accepting the ceiling as a valid call.