I admin a number of different websites. The majority of them are static. I have forced https redirect on some of them. On others I do not. The only benefit of https I perceive in the case of static public content is that ISPs cannot easily monitor which specific pages on my domains are being visited. With plain http they could. I don't particularly care if people get MITM'ed when visiting my static sites. If they did…
But even ignoring that, why wouldn't you want to take basic steps to protect against rogue access points or ISPs? "I don't particularly care if people get MITM'ed when visiting my static sites" seems, at least to me, like a rather dismissive attitude towards the security of your site's users.
I'd also like to point out that with the ever-widening deployment of HTTPS the hypothetical attack you described where your ISP MITMS your browser install is becoming less and less feasible. Nearly all modern browsers include a [HSTS preload list][3] which ensures they will never attempt to connect to certain domains over an insecure connection. A browser download site (or even just the user's search engine, which would link directly to the HTTPS-protected download page) being on this list would make the downgrade attack you describe much more difficult.
[1]: https://www.internetsociety.org/blog/2018/04/amazons-route-5...