1) Others knowing knowing exactly which package you are downloading could give them a very clear way to fingerprint and target services you are running
2) HTTPS could allow you to potentially reduce your risk surface in a case where there is a bug in Steam's signature checker itself and someone sends a crafted payload instead of game files. Server validation would have prevented that. This has actually happened with apt before: CVE-2019-3462.
Odds are small, but I believe in DiD. I could imagine an attacker with nothing better to do could get up to this on say, a Uni's network.
And before you come at me with I should be using a VPN to protect against these scenarios, I do that already. This is about HTTPS being a sane default when possible.
The default for package upgrades on PCs should be HTTPS (as it already is on Arch, Alpine, NixOS, Flatpak, and Homebrew) since it reduces risk with minimal performance impact for most users.
Or they could connect to the service.
> Server validation would have prevented that
No it wouldn't have. HTTPS does not ensure a server's contents are trustworthy.
> And before you come at me with I should be using a VPN to protect against these scenarios, I do that already
So you protect yourself from unlikely, irrelevant threats by... deliberately paying strangers to MiTM your traffic?
You publicly demonize something long enough and otherwise intelligent people start believing HTTPS is a magic invisibility cloak or the Good Housekeeping seal of approval for the web.
It is a tool in the toolbox and nothing more. Wearing safety goggles while making spaghetti is equally stupid.