There's no reason that pagerank couldn't be adapted to take into account wayback machine urls, there is a link with a url pointing at https://web.archive.org/web/*/https://news.ycombinator.com/ google could easily register that as a link to both resources - one to web.archive, the other to the site.
there is also no reason why that has to become a slippery slope, if anyone is going to ask "but where do you stop!!"