It is a very legitimate concern. All it takes is one cancer.

This is an entirely reasonable concern and I agree with it. If you're not in the US (or you're young / otherwise don't know people with horror stories) it may not make sense to you, and yes, it's awful that it's realistic, but it's still realistic.

1. Coverage by medical insurance, in the US, does not generally translate to 100% coverage. For a variety of reasons, including coinsurance, copays, out-of-network coverage (especially common if there's some sort of emergency), in-network coverage beyond what's covered (e.g., the doctor says "this is medically necessary" and some bureaucrat at the insurance company says "I disagree"), and so forth, you can have a medical insurance plan and still be on the hook for large amounts of money.

(I don't think you implied this, but just to clarify, "making good money" is typically not strongly correlated with having good insurance. Because of how byzantine medical insurance is, it's hard for a potential employee to figure out what their offered benefits will actually cover, so it's generally not a factor in negotiations at all, unlike actual salary, and in turn higher-paying employers don't have a particularly strong reason to offer better insurance plans.)

2. You can have a "life-threatening condition" that is easily manageable via modern medicine. Take diabetes, for instance - life-threatening if unmanaged, but very well-understood in how to manage it. I have a friend with a chronic condition that requires taking an (expensive, only partially covered by high-tech-employee insurance plans) injection once a month. As long as they take that injection they're fine; their body becomes effectively unusable without it. I know multiple people who have beaten cancer; I don't think any of them would say that the actual process of beating it was easy (or cheap). The whole reason you care about medical care is so that "life-threatening conditions" stop being life-threatening.

Also, we have this pandemic going around which absolutely can be life-threatening, but even among the people who get very sick with it, many of them come out of it fine provided they have immediate high-quality medical care.

3. Beyond the assumption that the person you're replying to is treating family poorly... the amounts of money in question are generally larger than can even be pooled across many family members and large support networks, even if they all have substantial savings on their own. And even if technically I can manage to pay off a medical bill by exhausting my life savings and those of my extended family, what happens when someone else in the family gets sick?

Coverage issues happen in EU with public insurance as well - years back one of my coworkers son had an eye tumour, the experts in country didn't want to treat him because they knew their equipment wasn't precise enough to treat it without blinding him, the medical insurance would not cover out-of-country treatment because the procedure could be performed in-country. He ended up asking for charity to pay for it out of pocket (can't cash out retirement fund). Another coworker needed to get a private surgeon for his father because he was old and the waiting list on cardiac surgery was months, he was not a priority because age/condition.

I don't understand how those amounts can be larger than you can pool from a support network if you have insurance. Even if you lose your job/insurance your spouse can cover you ?

My friend in graduate school (making $18k a year) had a baby who had some complications; the final bill was $1.5 million dollars. She was an immigrant from the Caribbean; her family there was reasonably paid by national standards there, but... $1.5 million is a lot to cover. And remember, the spouse can't cover you unless you're registered on their insurance.

A few weeks in the NICU and a couple surgeries and there you are, and there is no 'lifestyle change' that really can prevent that sort of thing.

Don't get me wrong - the situation sounds terrible and I'm sure the US health insurance system is pretty bad for a lot of people (at least it sounds like your friend got the treatment and can go through bankruptcy to clear off the debt) - but from OPs perspective, being covered by insurance on a good income - being in constant fear of medical bankruptcy sounds bizarre to me.

It is bizarre! We live in a country with bizarre healthcare structures! No one is denying that it's bizarre! We're just saying it's true. You Europeans cannot imagine what it's like here thanks to being surrounded by competent social support structurs your whole life, and it's beyond frustrating that you think you know better than we do how awful it is in our country and how we could "just" do something to make it better. If we could, we would.

I feel like if there's a risk of needing to go through bankruptcy in order to have a child, I would be justified in being terrified of it. Childbirth is in fact a life-threatening event, but it's also one a lot of people go through successfully, and you want to end up on the other side of it with enough money to raise the resulting child.