I still don't see what one has to do with the other(s). Besides, go browse through SO's unanswered questions; this is the state of things today! Simple questions, the most common variety, get answers in minutes. The complex and/or specialized questions which cannot easily be answered by a large portion of the userbase tend to languish.

Indeed. The result is that SO is a lot less useful than it used to be. I have repeatedly found myself chasing an obscure error message only to dead-end on SO and be forced to backtrack.

While I've experienced something similar too, I think it may have more to do with the fact that tech has diversified in the absolute sense. An obsecure question about your processor was easily answered in the past because everyone had tinkered with it. Today a question about a bolted-on feature in <insert tool of the year> might not receive an answer because so little experts on SO use it.(?) I've got no clue if this explains it, but it makes more sense to me than the idea that SO's attention/incentive span is so short that users are only spending time on 5-minutes-typing questions.

I've had that happen repeatedly as well, both now and "back in the day". I have no idea if a higher or lower proportion of the tougher questions are being answered now than they "used to be", but my sense is that it is about the same. The tougher questions are just tougher to answer; that hasn't changed through time. What I do know for sure is that a much higher proportion of easy questions can be answered by searching google and following one of the first few SO links, which is valuable.