The US will pay its debts in USD and the German government will pay its debts in Euros. If you think the euros to dollar exchange rate will be better in the future, it can easily dwarf the difference in interest rate between the bonds.

The point is that it doesn't matter, the US is toxic for the next foreseeable twenty years, and for Europe as a whole, a threat and an enemy. We'd be stupid to keep funding your economy, no matter how much money it makes back. Enemies are to be taken down.

>under this president and the one who replaces him in three years.

The levels of optimism in this sentence are off the charts. The US's political systems are so weak, fragile and compromised that you don't even know if you're going to hold proper midterms or if you're going to get a civil war, the current president is threatening the FED, but sure, debts are going to be paid when it's ran by the dude that managed to bankrupt casinos.

The risk for credit default is very close to zero, but the risk for renegotiated contract terms is not zero. That's what the whole Mar-a-Lago Accords was about. It is a strategy that describes in detail how that would be executed. The likeliness for which is up for debate, of course, but it's certainly not a risk free asset.

Add to that the rampant debt increase over many decades and zero political will to rectify the situation, which is why the rate of return is so much higher than in more politically stable countries such as for example Switzerland, Germany or Sweden.

This president has publicly mulled not paying its debts.

It's time to stop magical, wishful thinking about how you want the world to be, and deal with the world as it is.