I'll blame part of this on the societal attitudes that contribute to invisibility of people with disabilities, but the people who are saying this just really haven't done any research. Sure, people in some lines of work could pay off the costs of a broken leg, or eventually pay off cancer treatment. Even so, I'm guessing most of these people haven't even seen the hospital bills for this sort of thing. Inflated hospital costs aside, it costs a couple grand just to walk into an ER. My hand surgery that took about half an hour was in the tens of thousands. Yes, these kinds of bills fall into the "payable" category for many people.
All of this assumes that these people aren't ever going to acquire a disability that doesn't allow them to work. Unless you're one of the very richest people in this country, you don't currently have enough money saved up to pay for all your expenses for the next 20 or 60 years while you're living in a group home and taking a fistfull of meds every day that sometimes are successful in allowing you to form a coherent sentence or functionally complete a task.
Some of these people say that if they got in a horrendous accident and needed life support, they'd kill themselves. Putting aside how most of them can't possibly mean this and just don't want to part with their precious paycheck, most acquired disabilities don't involve long-term dependence on any sort of life support that one can opt to remove. If someone sustains a brain injury and can't work or live independently, there are absolutely no sorts of living wills or legal orders, pre-existing or not, that allow an individual or his/her guardian to choose to kill the person. A guardian also can't decide that someone without basic life skills can leave a facility and go wander around in traffic. The courts will decide that this isn't in the best interest of the person. You know those mumbling incoherent people wandering the streets panhandling? Those are the higher-functioning of the people who have brain injuries and/or psychotic disorders. Those are the people who have enough ability to eat and keep safe so that they're legal competent to choose to refuse treatment. A lot of people who have acquired disabilities aren't in any place to choose. But more importantly, if people really had good information about the disabilities they could possibly acquire, I really don't think very many of them would say that they'd prefer that if they get in a car accident, they aren't fed or bathed or provided with any sort of activities, since they don't believe in being part of the healthcare system.