Note you can still post answers to older questions; this is actually encouraged!
Closing of duplicates is the least of Stack Overflow's problems IMHO; it's done for a very real reason: if I search "how to do foo in bar?" then it's really great to have one question with all answers, instead of having to trawl through several questions. Also, as someone who spent quite a lot of time writing in-depth answers over the years, I'd rather not repeat that every month.
I have a lot of criticism of Stack Overflow, and I mostly stopped answering because I got fed up as well, but this isn't a problem IMO.
> Also, often enough the accepted answer for a question is not the best answer and instead the best answer can be found by looking at other answers or the comments (which I agree with the author should not be seen as ephemeral but are instead critical)
Generally those comments should be integrated in the answer; usually if I come upon an answer with helpful comments I will edit the answer to include that information.
I do wish the entire "accepted answer" feature would be removed though, or at the very least stop pinning them to the top.