The wisdom of holding is always relative to the rest of the market and what else you could be holding. Imagine a metals exchange. If copper were going up in value compared to the other metals, you’d want to hold copper. But you can also sell all your metals for cash. You would do this when all the metals are going down in value, which means USD is the “best-performing metal” and you want to be holding it. You always want to be holding the thing going up in value the most. Sometimes it’s USD, and the fact that it is also your reference point for pricing everything else doesn’t mean it’s not front of the pack sometimes. Obviously when you buy USD with your copper, the value of USD goes up. But since it’s all denominated in USD, that just looks like “all metals go down in price but especially copper”, and because so many other markets set the value of USD, it really just looks like “copper goes down”. It’s similar to the relative velocities etc you do when you study physics.
So primetime for holding cash is when there is deflation and you expect to be able to buy more for your dollars later, so you stash it in a bank account or under your mattress. This is generally bad for a regular economy because people stop spending and the economy slows down. That’s why there’s always a little bit of inflation, because better that than deflation! Inflation makes people use their money lest it lose value / gather dust sitting still, and spending money makes the economy run. Having to beat inflation with your investment is a chore, but it’s better that everybody individually decides to do something with their money. I digress.
For Tether, where the only things you can really buy are other cryptocurrencies, deflation just looks like a crypto bear market. So people are likely holding USDT right now.
At the same time, given that the value of USD is set by many trillions of dollars changing hands and millions of contracts priced in it every day, compared to Tether which is like a gift card for a record store that might go out of business, it’s possible for USDT to get unpegged if that record store posts bad results. The strength of the proposition that USDT can be redeemed for USD is the only thing holding it together.