Professor Garton Ash seems to make several assumptions about what will happen, all of which look unlikely or impossible.
Fiscal Union? almost impossible. They couldn’t organise it when things were going well.There would have to be referendums and a strong desire to do so in many countries.
There will therefore be a very much smaller Eurozone.
Joining a smaller eurozone will be the choice, not a EU with full political and financial union.
Joining the new Euro will be of even less relevant than it is to today.