Krugman has this to say about it:
For those not familiar with how financial markets work, US Treasuries are the ultimate safe asset, used as collateral for everything. Even a hint that some Treasuries might not be honored could bring everything to a screeching halt
Musk and Trump are both in the habit of stiffing people they owe money. If markets even suspect that this habit will extend to Treasuries, God help us
Why betting on stocks to fail is allowed in the US and elsewhere is beyond me, it always leads to this type of rich people trying to game the system to get even richer at the cost of everyone else around them. It’s a sign that the wealth-concentration and inequality is out of control + captured the mechanisms that should be regulating and preventing this from happening altogether.