This "complex" question of retroactivity in criminal cases

The U.S. Supreme Court is reviewing now reportedly a very complex question of retroactivity of its decisions pertaining to unconstitutionality of a "'catch all' part of the definition of violent felony, called the 'residual clause'". 

Now there is a reportedly VIROGOUS debate, by the U.S. Supreme Court, at taxpayers' expense, as to HOW RETROACTIVE that decision should be, how far back it should go, whether the U.S. Supreme Court's decision rendering a certain statute unconstitutional applies to ALL convictions based on that UNCONSTITUTIONAL statute, or only those "pending on collateral review".

Think about it.

A person is sitting in jail, and his or her conviction is based on admittedly-unconstitutional statutory law.

Yet, courts are VIGOROUSLY DEBATING whether the decision that the inmate's order of conviction was also unconstitutional - because it was based on admittedly unconstitutional statutory law - can be vacated, or whether the inmate should continue to sit in jail.

Every law student is taught in school the case of Marbury v Madison where the U.S. Supreme Court said that any law that violates the U.S. Constitution is VOID (not voidABLE, but VOID, as if it never existed, a nullity, a nothing).

The same says the Supremacy Clause of the U.S. Constitution.

So, it is a no-brainer, based on existing law and U.S. Supreme Court precedent to answer the "vigorously debated" question of "retroactivity" without ANY debate.

All convictions based on the law declared unconstitutional, no matter when they were obtained, must be vacated - because they are VOID, same as the statute of conviction of sentencing declared unconstitutional.

A "vigorous" debate is - should we vacate ALL unconstitutional convictions, or just SOME?

What are we paying those judges for?