People always talk about how SO is so terrible, and yet it's still the best resource by a mile. People have just forgotten that the lack of that carefully curated environment created the complete mess that was all the forums we used to have before. Sure, it's not ideal - SO could do better in terms of helping people understand the site's goals and enforcing the rules in a less hostile way, but they are working on tha…
I don't know, I've always felt that the apparent decline in usefulness came from the strict curation (and I do insist on the word "apparent"). In the past the site felt more natural to me as a big hodge-podge of various technology related topics. Questions came from various aspects of the tech world, whether they related to sys admins (e.g. linux configs, installations, scripting, etc), super users (e.g. text editors and other tools), programming languages (e.g. how to use a library or language feature), comp sci (e.g. algorithms), databases, or even networking. To me it all kinda related to how we all use the set everyday. Then came what I can only describe as over-categorization. Instead of relying on the machine to search stuff for us, they split the content into specific websites and changed their standard of what's relevant or less so (which is why you have closed questions that thousands of users still find useful). Google figured this out years ago with gmail, don't waste time organizing, just search. Case in point, how many people actually search any of the stackexchange websites directly? I'd bet that most users get there from google. And it's not unusual to be searching for a topic that intersects with any two or more of these communities. But none of them will refer to any other to tell you that the topic has a related thread in a sibling community. Only google gives you a clue, because it looks at it as the afore-mentioned hodge-podge.
It's not to say that the site didn't need any curation, I just think it somehow became a bit too systematic and oblivious of our instinct as human users. I also have the strong suspicion that people who feel that SO is declining are way off, but I doubt that the rise (if any) of its relevancy has much to do with the heavy handed curation.