There are probably a million different websites and applications hitting their HTTP endpoint, and likewise for scripts and assets embedded in their webpages hit

There are probably a million different websites and applications hitting their HTTP endpoint, and likewise for scripts and assets embedded in their webpages hitting third party non-HTTPS endpoints. If you just start serving your existing HTML and APIs on HTTPS, lots of stuff will break. They probably have more websites and hostnames than you'd realize, as well. Take a look at a site like https://dnsdumpster.com/ https://dnsdumpster.com/ and search for bbc.co.uk