Core could be opt out (or maybe not let them opt out) automatic updates, but plugins could (maybe) be opt in at best.

To many people run blogs with plugins made by hobbyists, or "theme frameworks" that break backwards compatibility on upgrade. An automatic update would hurt them too much. I'd be in favor of it normally: I think all professional software should automatically upgrade (maybe with an opt-out, but maybe without a choice is better) but that's it's not for the WordPress World. WP Core counts as professional, but most plugins/themes do not.