I had a look at all your references but didn't find any cost estimates or clear predictions of serious harm. One problem it mentioned was that already depleted

I had a look at all your references but didn't find any cost estimates or clear predictions of serious harm. One problem it mentioned was that already depleted shellfish stocks might not recover if we stop overfishing them. We're already doing more harm to those particular fish from fishing than ocean acidification is expected to, so that suggests something about the cost being comparable to the cost of fishing less. To use your analogy, put a stop light in the middle of the road, just in case a kid happens to run out at that point.