Affordable Care Act (ACA) outlawed denying insurance to anyone, regardless of how much their future healthcare expenses will be.
It is one of the reasons that everyone complained about ACA increasing health insurance costs - it had to because more people were getting more healthcare.
ACA also outlaws pricing insurance on anything other than age, smoking status, and location. Even the pricing due to age is capped so that the premium for the oldest (riskiest) age is only allowed to be 3x the young age premium.
ACA also implemented out of pocket maximum for in network care, so that there is a maximum cost per calendar year you would be responsible for. Recent law that went into effect Jan 1, 2022, extends this out of pocket maximum to all healthcare providers in the US if it is an emergency.
Finally, ACA also instructed an appeal process if you think the insurance company is denying payment for treatment justified with evidence:
This one is pretty messed up indeed. What's the purpose of being insured to begin with if it will fuck you over when you need the insurance?