I guess the difference in this case is that the JS on web archive relies on future browsers being backwards compatible, whereas irchiver relies on much less to stay timeless which is good. Although I don't think JavaScript will ever get a major update (as in breaking comparability) I believe relying on that is not a perfect way to archive web content. This kind of backwards compatibility breakage is something we have seen before with the deprecation of Adobe Flash and it could theoretically happen elsewhere on the web stack.

Agreed, I wish they would archive the DOM too like archive.is does instead of just the requests.