De-pegged is just any value less than $1. If USDT is 50% collateralized, maybe it is worth $0.50, which is de-pegged but not worthless.

De-pegged (I means really de-pegged, not a transient .97c a dollar on exchanges like it happened) means it's not fulfilling its promise, hence the trust is lost.

Also, Tether would try to save face, by honoring USDT redemption at 1$ as long as they can, which drains their reserves. So if they aren't able to do it anymore, it means that they don't have anything left, and USDT is now worthless.

These things are highly non-linear and bank runs can kill a bank in no time (and tether is functionally a bank).

Sure. People disinclined to trust Tether's word are already out of USDT. Clearly, a lot remain. Some of those people might attempt to profit from arbitraging a de-pegged Tether (maybe at $0.97 rather than $0.50 but it's the same principle).

We said we were 100% collateralized, actually it's only 50% but you can trust us that it really is 50% collateralized this time and we aren't lying again. Transact with trust!

The only stable values of a USD stablecoin are $1 and $0.

Perhaps as the mass USD redemptions start to fail, there is a window of opportunity to buy BTC before exchanges put a stop to it.