The individual mandate was intended to bring more people into the insurance pools and thus lower the cost of insuring sick people. (Failure.)
However, even without the individual mandate, insurance companies are still prevented from denying coverage based on pre-existing conditions (under Obamacare.)
So no one slips through the cracks. They never did really. During annual open enrollment, all major carriers have allowed pre-existing conditions for years and years.
Not so, and I know this first-hand. If you had a pre-existing condition, you could NOT buy health insurance, even if you had the ability to pay an increased premium. (Oh, you could, with a massively increased premium, if you just immediately lost your insurance, like timing out of a COBRA plan - but if you weren't in that exact position, you were S*** Out Of Luck.)
I think you should be able to buy insurance no matter what - to not have it can turn into a death sentence for some. I understand that for some it will have to be offered at increased premium cost, due to the known increased load they'll put on the system.