Personally, I wouldn’t go $100K into debt for any undergraduate degree, including any STEM degree. But I’m a little risk-averse when it comes to taking on debt.
There’s an argument that this is irrational, however. At most elite private colleges and universities, total COA now runs around $60K/year, and half or more of the undergraduates are full-pay. That means half the families are deciding it’s “worth it” to invest $240K in their child’s undergraduate education, regardless of major. That may or may not be rational, but I don’t see why it’s any less rational to say, “Well, I’d rather pay $140K over the next 4 years and spread the remaining $100K over more years by taking out $100K in loans.” Of course, you’ll end up paying more that way because of the interest rate on the loans, but that’s true anytime you borrow money, whether it’s for a new home, home remodeling, a vacation home, a boat, expansion of a family business, or whatever. And the interest cost is at least partially offset by the time value of money and inflation. Student loans or parent PLUS loans tend to have higher interest rates because they’re unsecured and have a relatively high default rate, but even so, it could be rational to take out loans if you expect your future earnings to be higher, or your future expenses to be lower (e.g., because you’ll no longer have Junior to support), or because you expect a windfall from a likely inheritance or sale of the family business or whatever. A similar logic would apply to families who are not full-pays; if you can’t pay $240K but you’re willing to pay, say, $160K over 4 years, there could be circumstances under which it’s rational to pay $15K per year now and to spread the remaining $100K (plus interest) over future years by taking out loans.
What is not only irrational but also cruel and unfair, IMO, is to expect to unload all that debt onto Junior, who will almost invariably have lower earnings at the outset of his or her career, and for whom the cost of servicing $100K in debt is likely to be crushing. I’d never do that to my kid, not for any undergraduate degree in any major.
Medical school and law school might be a different story. My impression is many newly minted MD’s have $100K or more in accumulated educational debt, and most seem to make it financially, though not without some penny-pinching in the early years. It used to work for lawyers, too, but given the current state of the legal market it’s now a much riskier proposition, even at elite law schools. Still, many students at the elite law school successfully navigate that.