At the end of the day, there's a trust issue.
The way distribution works nowadays, mitigates a lot of those. Making it decentralised would be a step backwards.
This is why torrents would actually work fine in this model.
What a nice gesture! However my mirror-servers are https plus rsync (for some projects). But are there not some ISP's that block torrent-traffic completely?
Well, that's how software is, but that article is from 2008 and things have gotten a lot better in the meantime (I think archlinux wasn't even signing their packages back then).
If the distribution's private keyring isn't compromised and you don't have third-party repos, your packages are as trustworthy as your distribution team (and upstream).