A lot of content which exists on the unmaintained web is going to be lost anyway, regardless of HTTP. Hosting sites shut down after a while. There's a ton of content that was on Geocities, for example, which is now inaccessible.
A migration of newer websites to HTTPS is no more a loss to data than deprecating old versions of HTML in newer versions of Chrome or upgrading newer servers to HTTP/2. The old ones will still exist. And if they cease to exist, it's almost certainly not going to be because a browser displays a "not secure" warning on the top left corner of the scene.
There are also entire websites and communities built upon the archival of data:
A migration of newer websites to HTTPS is no more a loss to data than deprecating old versions of HTML in newer versions of Chrome or upgrading newer servers to HTTP/2. The old ones will still exist. And if they cease to exist, it's almost certainly not going to be because a browser displays a "not secure" warning on the top left corner of the scene.
There are also entire websites and communities built upon the archival of data:
https://www.archiveteam.org/index.php?title=Main_Page
https://archive.org