I have always found that whenever an opinion cites a URL the courts are careful to indicate the date that it was accessed. A hard copy (or at least a PDF) of the page as it existed at that time is then retained by the clerk in the case file. There's usually a footnote concerning this arrangement.
It's not that hard. No need for fancy technology or mass archiving of the Internet. The only thing they need is a basic PDF writer. Problem solved.