Rather than holding US$ to back USDT they have been holding commercial paper ie. loans which pay interest. High quality corporate bonds pay something like 4% so say they invested their $72bn in that they'd be making $2.8bn a year for doing nothing.

They may have invested in iffier stuff but they are on such a nice earner they will be quite motivated not to break it. This is not like Madoff when he was paying 10% interest that was unsustainable. They are paying 0% interest which is not so hard to sustain.

If something gets them it'll probably be anti money laundering regulations, not running out of money.