Regardless of the fact that I disagree with your proposal, I'm always grateful for ideas and willingness to improve the site, so thank you very much for posting, and thank you so much for the huge amounts of reviewing you do too.
I guess I have just enough to add to post an answer, though I strongly agree with other points that have been raised.
While poor quality answers may be a problem, I see them as a less bad problem than questions having no useful answers. I firmly believe that old questions deserve just as much attention as new ones, because most of the people who are helped by this site, I assume, search for their problem and find the answer without ever posting anything (although it's likely the engaged community is helped by this site much more per person, in various ways, than those many visitors). Searching and landing on unanswered questions, or questions with only out-of-date answers, is frustrating. I think we should do our best to encourage people to answer questions, both new and old.
There's certainly a moderation overhead in allowing new users to post answers everywhere, but the only reason that overhead may be slightly higher for old questions than for new ones, in my opinion, is that new users are likely to answer old questions (perhaps as likely as they are to answer new questions), because (judging by the better quality late answer posts I see), finding a solution to some problem of their own may be what motivates them to post on the site for the first time (or first time in a while). I think that is a good thing. We need new answers, and we need new users too.