> "Reduced security" is not a valid fallback option.
Agreed! But if my HTTPS is broken, I might well want to replace my site with an HTTP page explaining that we'll be back soon. If that is impossible until the max_age expires, that can lead to an awkward explanation to the higher-ups.
1) You're not going to be able to do that for anyone who has bookmarked the site, or loads it from their history / address bar, with the https already included. Under what circumstances, other than someone hand-typing a URL, do you expect anyone to reach your site by HTTP? (And note that any such user can potentially get compromised, such as by ISPs.)
2) Search engines will link to your site with the https pages they found when crawling. And if you stay down long enough for search engines to notice, you have a bigger problem and a much more awkward explanation to give.
3) Many kinds of failures will prevent anyone from reaching the whole domain, or the IP address the domain currently points to, or similar. Have an off-site status page, social media accounts, and similar.