Regarding your last paragraph: Isn’t that pretty much solved thanks to HSTS preload? A non-technical author of a small recipe blog might not know how to set it up, but a bank ought to have staff (and auditors) who takes care of stuff like that.
Are there any real world online resources where, modulo redirect, a different resource is presented on the HTTP and the HTTPS protocols? Or alternatively, on ports 80 and 443?
There used to be, though it's less true now. However, the reason to treat them distinctly (as different origins, technically) is that HTTPS provides integrity whereas HTTP does not. So, consider the case where the client enters an HTTP URL and is redirected, just as you say above. If the attacker injects their own JS and it is cached in an origin that is just `example.com`, then they control the user's experience of the site, even if later the user securely goes to the site with HTTPS.
Thank you. That really is a novel attack that I didn't think of.
It doesn't solve the problem of a client having to treat https:// and http:// URLs with the same string after the :// as distinct resources.