*References handy cumulative probability distribution*
Give or take (due to variations by year for outlier claims, as well as variations by age, gender, and region)... About half a percent of people in any given year have total allowable claims exceeding $60K. On average, insurers negotiate a discount of about 40% off of billed prices... so that means the actual incurred cost is about $110K.
I don't think many people could afford to drop $110K for a major accident.
So let's drop the bar... let's say you think $20,000 of personal money is something that "a lot" of people can afford to shell out in the event of things going terribly wrong. That's an average allowable cost of $12K (after the negotiated discount of the insurer). At $12K... it looks like about 5.5% of people hit that amount.
So do you think that 5% of people can afford to drop $20K in one year for medical services, without seriously untoward affects on their quality of life?
How about $10K? That's not far from the OOP specified by ACA, once the discount is taken into consideration - it's about $6K with the discount, and ACA specifies an individual OOP of no more than $6350. That puts you at 12% of people.
So... 12% of people in any given year will incur medical cost commensurate with the OOP specified by ACA. Expecting someone to have $6350 in their bank account, untouched for any other reason, is probably already asking a lot.