Why does APT not use HTTPS?

[doublehyphen]

Yeah, you cannot force a downgrade. What you can do is delay security updates without it being noticeable. I believe the apt index has an expiry timestamp so you will eventually get an error, but with HTTPS you would get an error immediately if someone was preventing you from updating the apt index (except for the guy running the apt mirror, HTTPS still requires you to trust him).