Following that logic, any obvious ponzi / pyramid scheme is credible and trustworthy until it demonstrably collapses?
You say tether is "maintaining" the peg but that information comes... from them. They are not audited. They are unregulated. You don't even know who is behind it. You won't see that they are not even close to solvent until a bank run occurs. It is easy to suspend disbelief as long as nobody requests their fiat from tether in cash in large volumes. In those huge volumes, no fiat is changing hands, so they can very easily run with no cash at hand as long as people believe they have the money.
When I looked at it in more detail a few years ago, they even claimed you are not entitled to the fiat in exchange to the usdt you buy, is that still true? Is there even any way to get any fiat from them directly? Sure, you can convert your usdt to fiat using an exchange by buying another cryptocurrency first - in this chain everyone "believes" tether has the money to back it up and that's why it works. It could work even if tether had absolutely no usd because there is no reason to touch that usd.