> taught in business school

The school is ethically wrong here.

Now we’re back to the opening comment: who’s job is it ethically to ensure people receive a living wage? The companies that employ some people or all of us?

Most people don't have leverage to negotiate a fair compensation for people to do it on their own. So you're presenting a a false dichotomy. "all of us" includes the companies that employ "some people". For better or worse, America seems to reject the idea that "all of us" means the government should help people out through taxes. That therefore leaves that companies must do it.

Just realistic, not idealistic.

Exactly. This has nothing to do with ethics and the school wasn't wrong from an economic perspective, which is what you learn about in, ah, business school.

Furthermore, there's a presumption of ill intent - a question begged: do all employee replacements occur because the employer unethically fires their employee?

Get a broader education.