Yes. Most people who are employed full-time are expected to work full-time.

Yes, they could leave full-time work and fine new sources of income, but that no longer falls under "just work 30 hours instead".

This doesn't mean that you are suited for 40 hours a week.

It just means you have no option but to overwork yourself.

Correct. It's not an option.

You keep using that word. I do not think it means what you think it means.

There are next to no stable part-time jobs in programming. I don't really know what you're getting at.

There are next to no stable part-time jobs that can sustain someone above the poverty line in most fields.