"I suspect that the gradual decline, beginning around 2016, is due to growth in a number of other sources of answers."

I think at least one other reason is that a lot of the questions were already posted. There are only so many questions of interest, until a popular new technology comes along. And if you look at mathoverflow (which wouldnt have the constant shocks from new technologies) the trend is pretty stable...until right around 2022. And even since then, the dropoff isn't nearly so dramatic. https://data.stackexchange.com/mathoverflow/query/edit/19272...