[kiliantics]

Usually a professor gets a grant with money specifically allocated for paying grad students. It's not like there's an opportunity cost involved since there's nowhere else that money can go.

A graduate who spent the 6 years getting experience in industry instead of doing their PhD can expect to be at a similar salary level as the person being hired out of the PhD. They also earned more through those 6 years and so they are on average better off overall.