The only way automatic updates would be anything but a disaster is if it was manually opt-in, instead of opt out.

Not only do web developers often mod themes and plugins directly, instead of via a child theme, even non-programmers can find on the web how to make little changes, and are likely to do it directly to a theme or plugin.

There would be websites breaking all over, if things simply updated without asking.

There is already too much of this sort of busybody thuggery in the technology world, with operating systems that update themselves or programs without asking by default. It's OFTEN a cause of problems, as well as keeping people more ignorant of the workings and status of their own machines.