Stackoverflow works great for me, and I don't know why more people don't appear to use it like I do. 1) When you've got a problem, Google it. 98% of the time the answer will be in top 5 links to stackoverflow or documentation. You upvote the answer and question. 2) 1.5% of the time, stackoverflow will have a related question, but either it doesn't have an answer, or the answer has issues. You figure it out yourself,…
If most of the common questions have been answered, then there will obviously be a correlated decline in the number of questions posted and subsequently answers posted. Esoteric questions aren't abundant, especially over a constant time frame.
Declining question/answer activity shows that the website is healthy. I've opened S/O a few times over the past year with the intention to answer questions, only to find duplicate/off-topic/low-effort/etc. questions.