This is neat, however I'd like to see it archive the comments as well. It looks like it redirects to the actual discussion page on Hacker News, which is not an accurate glimpse of that thread at the point in time it was archived.
I imagine something similar must happen with Hacker News itself. We ask the owners for features, but not all of them are adopted (or implemented as soon as we wish). Eventually, some of us create companion products or services to scratch our respective itches.
I could possibly be convinced to adopt the feature you request if someone else were to implement it. First, though, there are some additional factors to consider beyond my time (factors such as resource usage on my webhosting service and whether Hacker News would be cool with this level of site scraping.)