HTTPS-everywhere, along with constant monitoring and reporting of certificate chains by the browser, are designed to protect against QUANTUM attacks [1], which, ~10 years ago, was being scaled to support million of simultaneous attacked devices.
HTTPS only is a "Fail Closed" system, ie it blocks access in case of failure. This is safe for the general population.
HTTPS/HTTP mixed support is a "Fail Open" system, ie it allows (unencrypted) access in case of failure. This is unsafe for the general population, see QUANTUM (above).
You can argue for wearing a bulletproof vest at home if you're an iraqi nuclear scientist. But for most people it doesn't make sense and does more harm than good.
In the same way, HTTPs only, *requiring* a system that "fails open", is bad for the general population. HTTP+HTTPS, yes, definitely. HTTPS only, no, only for sites and contexts where the rigid security is justified.
It is not cargo-cult security.
[1] https://en.wikipedia.org/wiki/Tailored_Access_Operations#QUA...