I'm most worried about the "long tail" of often very interesting, useful, and rare content (a lot of it from a time when the Internet was far less commercialised) that is unlikely to be hosted on HTTPS, and whose owner may have even forgotten about or can't be bothered to do anything about, but still serves a purpose for visitors. The "not secure" will drive a lot of visitors away, and even lead to the death of many such sites.

Imagine someone who knew enough to set up a site on his own server a long time ago and had left it alone ever since. Maybe he'd considered turning it off a few times, but just couldn't be bothered to. Now he suddenly gets contacted by a bunch of people telling him his site is "not secure". Keep in mind that he and his visitors are largely not highly knowledgeable in exactly what that means, or what to do about it. It could push him over the edge.

...and then there's things like http://www.homebrewcpu.com/ which might never have existed if HTTPS was strongly enforced all along.

I understand the security motivation, but I disagree very very strongly with these actions when it also means there's a high risk of destroying valuable and unique, maybe even irreplaceable content. In general, I think that security should not be the ultimate and only goal of society, contrary to what seems the popular notion today. It somewhat reminds me of https://en.wikipedia.org/wiki/Slum_clearance .

(I also oppose the increased centralisation of authority/control that CAs and enforced HTTPS will bring, but that's a rant for another time...)