If US debt-to-GDP is such a problem, why aren’t US Treasury rates surging and other fiat currencies surging relative to the dollar?

It’s a fragility problem. It’s fine until it suddenly isn’t. The higher the debt-to-GDP, the worse the impact of the same drop in GDP on debt financing. Easy to get into a bad spiral. 

(CHF-USD is up 10% YoY btw, but that has multiple causes)

OSS + Rust. I have one goal in life: the ability to stop thinking at will

Markets do though if you look at 10 year bond yields. Anyway, not my main point.

https://www.bloomberg.com/markets/rates-bonds

The Swiss are participating in the USD swaps program and just nationalized one of their two largest banks. I would not consider them a paragon of financial prudence.