Secure tenured positions are very competitive globally. There are only sufficient such jobs for a small percentage of PhD graduates.

Further, the university sector may be on the verge of massive disruption - look at startups such as Coursera and Udacity. Its difficult to be sure it'll provide reliable employment by the time you reach 65.

I've just finished a PhD program in an Irish university, which I got a lot out of; but I wouldn't do it for job security, or earning power.

But maybe you are talking about doing a PhD in a best-in-world institution; if so, maybe the picture is different.