Open source tools already exist. Wget can and is used to build out static copies of websites. It's also possible to get most of the benefit by using a reverse proxy cache with super long TTLs and a targeted purging strategy to handle content updates.

If you're tempted to point out that those approaches are a pain to use and don't work perfectly, well a) welcome to a lot of open source, and b) that's why commercial ventures can still succeed.