How to deal with the "pre-existing conditions' problem

If the SCOTUS removes the insurance mandate, they are very likely to eliminate the Guaranteed Issue.

Guaranteed Issue means people can't decide to refuse to cover you for a pre-existing condition - or (almost as bad) - to cover you EXCEPT for the pre-existing condition.

Currently, in the US, all health insurance sold in the small group market (usually employers of 2-50 employees) is already guaranteed issue.

I suggest a simple rule:

Give Guaranteed Issue  to anyone that has never been without insurance for a period of 2 months or less, starting at the time this rule goes into effect.

That in effect means you can't deny coverage for someone or even for a particular health issue as long as they have continuously held insurance from the law passage.  I threw in a grace period of 1 month to make sure that simple screw ups don't kill someone's insurance.

It also means that every single American will have Guaranteed issue for a period of 2 months.

Yes, it will put a strain on insurance corporations for a short time as they are forced to cover sick people, but it will also give a huge intake of healthy people getting insurance right away to make sure that they will always be eligible for coverage.