Just a thought, but couldn’t the insurance plan a person was on, when the condition was determined continue to pay for treatment of the realized risk, even if the subject leaves the plan? It may change pricing and add some complexity when conditions overlap, but this is one possible way insurance can work without forcing insurance companies to take on new policyholders and their pre existing conditions.
To make it easier, once an individual is in a plan, they should be able to stay with the plan, even if they terminate employment, however they must pay full premiums(non employer subsidised premium), if the terminate. Subsequent employers can give an allowance equivalent to their new term of employment health care subsidy to help subsidise the premiums of the previous plan.
There are also laws (COBRA) that require Insurance companies to provide coverage for insureds are separated from an employer related policy for 18 or 36 months depending on the details. This is so that they can secure other coverage before the current coverage runs out.