I was looking for the same thing, so I built it. It's a WordPress plugin that outputs a static copy of your site to a ZIP or a directory of your choice. The thought being that you put WordPress on a subdomain (e.g. wp.example.com) and have the main site (www.example.com) served by the static copy.
https://wordpress.org/plugins/simply-static/
It's still a work in progress (it launched a little over a month ago) but the feedback so far has been positive.