For early retirees, there are three main benefits of ACA--guaranteed issue, tax subsidy, and lower premiums for older customers than would be available in a market with underwriting.
As we transition to a post-ACA world, these benefits will change, worst case being we simply go back to the individual market that existed before ACA. I would be curious to hear from long time early retirees who actually shopped for and bought pre-ACA insurance, what that experience was like. I'm sure it varied a lot by state, age and how healthy you were.