There's a ton of important software that people are installing over HTTP, so using HTTPS is unfortunately already super-substantial progress. Chris Palmer gave the sad example of PuTTY a couple of years ago:
https://noncombatant.org/2014/03/03/downloading-software-saf...
(after what I think was a long time, the actual download links themselves are now HTTPS, although they're all still served -- along with the signatures -- from an HTTP page)
I'm certainly not going to defend the idea that HTTPS is enough authentication for software installations (I'm writing an article related to software transparency), but there's a pretty big bootstrap problem and infrastructure gap right now.
