I disagree. In math, I had a survivable wage from TA/RA gigs during my whole graduate career, and I escaped the experience with less than a year's (grad school) income in loans taken out in my undergrad. Saw the writing on the wall, networked a bunch, and found a good job and now I'm making more than my supervisor ever will. Could I do the work without what I learned in my PhD program? Maybe I could have stopped after a Masters... but then I'd have missed the networking opportunities and (given the options available after my Masters) I'd have settled for a job with much lower satisfaction.

On the other hand, I feel like I've defined myself in a narrow specialization, so I might miss opportunities because they don't fit that.

But definitely don't get a PhD if your plan is to become a professor. Unless nepotism will go in your favor.