Once the people choose to no longer follow the EU Supreme Court, thus, as a follow up, no longer choose to follow the EU treaties, they exit the EU just as the UK did.

No country is or can be forced to be a EU member.

Just because there are economic implications does not mean the EU treaties are above the countries‘ constitutions the people actually chose to enact.

It's a bit more complicated than this, because the treaties don't really have any mechanism for unilaterally expelling a member state and there is no precedent for doing so. The reality is there is no easy answer to what happens when a national constitution is incompatible with EU law.

Or, leave both the constitution and treaties in place and wait to see if the EU bothers to take adverse action.

Right now the political and economic interests of Germany and France is a strong EU. Won't always be true, though I can't foresee the circumstances when it might change.

Likewise, for most of my life, my personal political and economic interests included a strong UK (still does even though I moved to Germany) and USA even though I never lived there.