I've actually been working on-and-off for the last year or so to make a site which makes it easier to keep up with webcomics. As a part of crawling the sites to see if there are new comics, it's pretty easy to also archive them to be able to show partial thumbnails.

Part of my motivation is similar to your worry that these things will disappear forever, despite having been a huge part of our culture. And I figure one of the best ways to get help on archiving comics is for people to complain "hey your site doesn't do comic X" and then I know that it exists and can get it into the system for crawling.

I know saying "hey I'm working on it" isn't much better than "hey I thought about that too" but if you're interested I can attempt to let you know when I get far enough to put it up on the web instead of my workstation at home.