Opt-out for new installs, opt-in for existing installs.

The main objections seem to be from power users and from people who've had upgrades break sites in the past.

Power users - you are smart enough to know about the option and configure it the way you want it (as you do with the rest of your site), and hopefully you will recognise the broader benefit to the internet if more sites are kept up to date. Stop looking at it as something that is enforced on you and instead look at it as a choice that will help others.

Past breakages - those sites that broke from an upgrade would break whether the upgrade was manual or automatic. So the only difference is that it might happen when you aren't on hand to fix the problem. Solution: scheduling options for the automatic upgrades. And if you're concerned about clients, then you can always just turn it off and own the upgrade responsibility for those sites (as you probably already do).

I don't know why this issue continues to be so controversial. Automatic upgrades have done wonders for the security level of numerous operating systems and applications over the years. Time for WordPress to join the party.