This is a suggestion.
Why doesn't stack exchange automatically archive links in questions and answers. There are links to websites that no longer exist, etc.
Since they are such a large part of the quality of our answers, it would make sense to archive the links with https://archive.org/ or http://archive.is/ so that if the original link goes down there is still a record of the webpage and information there.
Perhaps stack exchange could offer their own archiving service.
Perhaps something where as long as the original link is live, the link goes to the live webpage, but in the event of the link no longer working, the cached version of the website is shown.
@msh210 pointed out in the comments that this had previously been suggested on the main Meta and rejected here. However, I think that for a site such as ours, where we emphasize sources so much, it would be beneficial to incorporate such a measure, more so than other Stack Exchange sites.
As an example, One of links in my answer to this question has on twice 404ed. I was able to find a replacement, but it was not ideal (the link used now is a copy-paste of the article in a forum): https://judaism.stackexchange.com/a/11898/603 -- As mentioned in the comment there, the article itself is a summary, so it wouldn't really be feasible to summarize it further. The only other option would be to copy and paste the whole article, which is also not approved by Stack Exchange.
Just found an announcement from Wikipedia and Internet Wayback Machine telling us that they've done this for Wikipedia: link
archive.orgarchives pages on demand, SE could automate a request to them any time a link is added to a post.