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.
I had a pre-existing condition (debilitating migraines) for decades. I was never once refused coverage.