[manholio]

This economic opportunity is only true for PhDs in CS or engineering and even there, only for hot research areas. Good luck getting 5x academic pay with a PhD in geology.

Generally speaking, the professor's advice from 25 years ago is still solid: don't get a PhD unless you are interested in an academic career, research, teaching etc. Due to the prestige of such a career, it tends to have an oversupply of applicants - the majority of which will not get a commensurate payback for the efforts required by a proper PhD thesis. So they will either flunk / present a low effort thesis, or worse still, they will invest a few years into a good thesis but never develop their career and skills gained into a full academic job.

So getting a PhD for the sake of it might not be a good investment of your time and effort career-wise; If you want to do it for the intelectual challenge on a topic you are very interested in, that's always a suficient motivation.