I think it depends on what you mean by “a good idea” or “makes sense”.
If you’re comparing a PhD to other professional degrees in high-paying fields like pharmacy or medicine, the PhD is almost never going to come up on top if you’re just looking at salary. It’s also difficult to make a direct comparison because PhDs don’t necessarily lead straight to a specific field. If you get a PharmD, chances are you are going to be a pharmacist, and we know how much pharmacists make on average (around $122K). If you get a PhD in chemistry, there are a lot of things you can do, some of which may pay more money than being a pharmacist, and many of which won’t,
There’s also debt and opportunity cost. A PharmD is going to put you in a whole lot more debt than a PhD, and so will an MBA or medical school. Physicians make a lot more money than PhDs on average, but for the first 10-20 years of their careers, their loan repayment may end up bringing their spending power closer together than you’d expect, depending on what that PhD is doing. And MBAs may not make much more money than a PhD. That just depends on where the MBA came from and what kind of work the person does afterwards.
On the other hand, there’s an opportunity cost to doing a PhD. MBAs only take 2 years and then you’re earning money. PhDs take 5-7 years or more. So that’s 3-5 years that you’re not earning money that you theoretically could have.
Yes. You are missing the fact that humans aren’t completely rational economic actors, and don’t make decisions purely on the basis of financial concerns and future salary. Interests, passion, job satisfaction, and career growth play into that. I have a PhD, and I was well aware of the fact that I had the potential to make more money had I gone to (and finished) medical school. But I didn’t want to be a physician.