Sure, the downvotes data is in the post you linked, but here's the data for both for the last year:
The upvotes data looks pretty healthy, but we can probably attribute some of that to the increase in traffic coming from Google.
And here's the percentage of questions that are closed or deleted:
Chris addressed most of your questions well, but just to add something on the topic of duplicates:
As more and more questions have a direct answer on the sites. More and more questions will get a down vote with a "Duplicate" stamp on them.
Closing as a duplicate is one of our least used close reasons. I don't think its use has had much of a negative impact on the amount of engagement or discussion taking place.
Partly, this is because our search system is currently pretty bad at actually finding duplicates, which is being worked on.
Theoretically, when a question is closed as a duplicate, it should be a very healthy close - it's saying to the asker, "This other question is the exact problem you're having, your solution is over there, enjoy!" - it's for the kind of question where an astute answerer would just copy-paste the answer from the existing question, so there's no reason to re-hash it.
In practice, a lot of our dupe closes are against our canonical questions, and not just random copies of the exact same question.
Running out of questions remaining to be asked is definitely something that could make it very hard to get involved in participating, but I think we're a long way from there - and our field is changing quickly enough that there should always be some fresh topics, even if we manage to reach the point that we've answered every possible Apache question under the sun.
The problems that we're having currently are a combination of poor quality questions and a decreasing number of people contributing answers. I'd love to be at the point where we're running out of problems to solve, and Stack Overflow might be there on some topics, but I think trying to solve that problem is just theoretical at this point.
