I don't think auto opted out for extant installs would help the matter at all.

If automatic updates is a must, then make sure a lot of people try the update safely on actual production sites before pushing the update out to everyone else…

Y'know... we do. A lot of us do (I'm running SVN on ipstenu.org right now, and usually do a cron job to svn up every night - I haz crazy). It's never enough, it will never be enough. It's absolutely mathematically impossible for the beta brigade to test every possible scenario with every possible plugin, server version, PHP version, SQL version, and fancy snowflake that is your install. But let me spin this out:

WP 3.4 (the first auto-upgradable version) is pushed and we all jump on board.

Oh wait, there's a bug. People scream in angor and raaaaaage! WordPress sucks! It's terrible, why are you all dillholes?

Someone makes a trac ticket. Otto and Nacin fix it.

Magically your site is fixed without you having to do anything.

Why ... it's just like the hotfix plugin, only I didn't have to do anything!