Because nobody cares about this particular thing. Those who understand that when international trust in the dollar falls, the US is in trouble, have been warning about this since months. The valuation against other currencies is rather a knock-on effect than a problem in itself; the dollar was at 1.50 EUR about 15 years ago and it wasn't much of an immediate issue. In fact devaluation can make sense if you want to strengthen your export. But it only works if other economies believe in yours, and this is where the issue lies.

I'd guess the Euro would be the most viable reserve currency as while it does have issues, the central issuing bank isn't beholden to a single government, and the economy behind it is quite strong. China might be a bigger economy (or not, I haven't checked numbers), but not even BRICS members apart from China itself use it as such as they understand reliance on China is a risk.

If the dollar is replaced though, the US will fall on hard times as you say. But at least the people that some were trained to hate can't use public bathrooms anymore, so all is well. And that's way more important than any semblance is a functioning country. And as such, they wouldn't care if they understood