Hi, I'm the plugin author. Piet was kind enough to mention this post to me.
The ability to archive sites wasn't my original intent when I created Simply Static, but I've had quite a number of people tell me that that's what they were using it for. That may account for a slightly lower number of WordPress installs than it might otherwise have, since shutting down WordPress would no longer could it towards an active install. And, more likely, it doesn't have more usage because I've done basically zero marketing of it.
Saying that you could make your WordPress site "hack proof" may not be the best phrasing. Hack resistant, perhaps? In any case, you could definitely lock down your WordPress site far more than you could otherwise, since only people contributing content to the site would need to be able to access it. And, as noted, if you end up shutting down WordPress after using Simply Static then there's definitely no WordPress site left to hack
Anyway, I'm happy to answer any questions you might have. My involvement with the plugin has slowed over the last couple months (exceedingly more busy with the day job) but it's definitely something I plan to continue supporting for the foreseeable future.