Nationalization is hard, every country is it's own thing and Germany is a US colony. But yes, EU / EUR is unlikely to go away at this point, it's wiser to stick together.
As the US fades from prominence, control over Germany is - ironically - likely to fall into the hands of Russia, which will make it their anchor in the EU.
> sustained third world decline
Well, once you take infinite dollar debt markets out of the equation, we're going to go back to a world where buoys float and stones sink. So countries are going to get the hint pretty quickly and adapt accordingly...
Can Europe become productive again? Sure, they've done it thousands of times.
Can they build a navy? Probably, but should they? Unclear. It's looking like big ships are big sitting ducks.
American protection was a double-edged sword, Europeans had to entertain American ideas of morality -> e.g. importing migrants against their best interests.
Lose NATO, lose the USD system, people who work and produce things will probably benefit no matter where they live. Big losers will be people who produce nothing and live by their political opinions.