I agree with doing threat modelling, but here's where I think we diverge: I, along with the EFF, think that HTTPS should be the "default" choice. You seem to think that HTTP should remain the "default" and that HTTPS should only be used where expressly justified.
If there are legitimate technical reasons to use HTTP then use HTTP. However HTTPS should be on most sites or services unless you have a specific technical, legal, or otherwise reason not to do so.
Keep in mind that every HTTP site you visit at Starbucks can be MITM-ed. So if you're visiting a HTTP site and then hitting a "login" or "contact" button which redirects you to HTTPS one then for all intents and purposes the entire site is now not MITM protected since an attacker can change that link to point to an insecure domain they control. HSTS won't even protect you from this if you're using sub-domains (since the attacker controls the link itself, so can link to another site they control).
Another popular example is: What if someone sitting in Starbucks is looking up AIDs information or something else potentially damaging to their reputation? If all of the federal government's web-sites are HTTPS by default then that little AIDs informational site is likely HTTPS also, however if HTTPS has to be justified using some kind of threat model, then more than likely it won't be HTTPS as it doesn't contain the kind of content people typically protect (just static HTML pages and images).
So in the threat model are you going to examine the possibility of reputation damage and complex phishing scams (e.g. replacing a government department's phone number)? And if you do look at it at that level are any sites really going to be able to justify not being HTTPS?