Looking at that graph though, what it tells me is that stackoverflow was entirely unaffected by ChatGPT, and the giant spike is just humanity collectively jumping on ChatGPT in addition to folks using Stackoverflow in exactly the same way as before?

Same for that "answers over time" graph: that's literally what you'd expect for a knowledge base. There has to be an initial trend upwards as the site starts to accumulate information (i.e. questions with answers, not just questions) and initially gains popularity as a place to get help by asking questions. But, as the number of questions and answers increases, the utility changes from a place to "get help by asking questions" to "a place to get help by searching for answers". At that point, the curve levels off, and as the number of typical beginner and intermediate questions die off because they've already been asked and answered many times, the number of new questions has to trend downward. It'll be slow, because we're constantly inventing new programming languages, libraries, frameworks, etc, but the majority of questions for those are simply duplicates of things asked about previous languages, libraries, and frameworks, with their answers being perfectly usable by folks looking for answers.

And this is exactly the graph we see: rather than a sign of decline, it's a graph that shows the change from a predominantly Q&A site to predominantly being a knowledge repository.

ChatGPT will, of course, have some impact on the graph, but the number of posts on Stackoverflow today that are "How do I X? I asked ChatGPT but that didn't help me" is staggering. ChatGPT may have initially driven down searches across the rest of the web, from Google to Stackoverflow to your mom's recipe website, but the novelty's already worn off, and overall it's done precious little to cut into the utility of Stackoverflow, as far as this data suggests.