A couple of years ago I would also have said that a PhD is not worth the opportunity cost. I look back to the the time when my wife and I were both fresh out of college and in our PhD programs. We lived _very_ frugally. After housing, food (all cooked ourselves, no take-out or eat-out), transportation (had to live outside of the city), we had maybe $100-$200 left over each month (in 2000-2004 dollars). We had no money to set aside for retirement or investments. We did take out interest free student loans and put them in a CD and paid them back immediately after we graduated, so that helped provide a little investment income. We worked hard to scrape by on a poverty-level stipend without going into debt.
Shortly after we got real jobs, I looked at our friends who went to industry right after college and they were significantly better off, and continued to be better for a quite a while (that whole compounding rewards things).
However, now that I'm senior in my position (real senior, not SV senior label), I have much more stability in my job than my non-PhD friends have. The PhD is pretty valuable for the work I do, and that keeps me as the lead on most of our contracts.
I can't say if I would be better of right now if I went to industry right away and started making money and savings, or if the stability has put me in a better place.
I don't have the massive income as an SV tech job, but I have also never been cut at a layoff, so I don't have to worry about the variability in income.
As an aside, I will say that a PhD is non-trivial and you should probably only consider it if you _love_ the field. If you're just using it to buy yourself some time or to live off the stipend because you can't find work, you probably won't have a fun time.
It is problematic to view it purely from the perspective of money. Consider the huge non-financial opportunity cost to not doing a PhD. If you get a PhD, you still have a chance of founding a business and becoming a multimillionaire. If you don't do a PhD, you'll almost certainly not get the benefits of the PhD ever.
I look at my friends who went straight to industry after their undergrad, and compare with myself and those who did a PhD, and there's a notable difference. If your advisor is not a slave driver[1], you'll have more time to introspect and be exposed to many perspectives/disciplines, at high concentration, than you have time for with a full time job. You can do it after work, but only if you opt not to have a life. It will takes perhaps 10 years of consuming all your non work time to perhaps match up 5 years of grad school. Pretty much no one goes this route.
Put another way: If your son discovered he had more earning potential if he never completed high school, would you say "Go for it!"?
[1] And contrary to what you read on HN, many are not.