Web-of-Trust fixes most of the "trust" issues raised (ha!). If a 100 people archive a web page and one archived version is different, the one version might be a forgery. If people who are "enemies" in the WoT sense have the same version archived it's pretty likely that it's legit. Yes there's Sybill attacks and other issues blablablabla but I'm pretty sure it would work in practice, and it would work pretty well.

But web pages are often served differently to different visitors. There can be location /personalization variations of exactly the same page. Even to different logged-out, “anonymous” users. Also, a “page” is more than a single page, there are usually dozens of associated files like css, js, and images. Just saving the html won’t get you very far when you want to look at it again later.