Life is short and the basics of survival require daily purchases. So while someone can take a cruddy job while still looking for a better job, all that time does add up to the opportunity cost of going for a PhD. Agree, it’s very much an open question.
I'm not sure it has to be immediately, but your point still stands, and it's often ignored by proponents, which is that merely acquiring a graduate degree does not automatically guarantee all those intangible benefits.
One has to actually secure employment with those benefits. Whether that's easier than finding employment with salary benefits instead (or in addition) seems to be still an open question.