That only helps if the treasuries get redeemed by then (and it also isn't quite what people would expect in terms of liquidity, I suppose. Kind of similar to other options then, e.g., looking at some recent situations, FDIC stepping in also doesn't seem to take forever for retail).

That’s exactly the point. If you have up to X days, you hold US treasuries with maturities of up to X days.

You can also add clauses to the contract that allow earlier redemption into the underlying securities with redemption less than X days.

There’s a lot of ways to skin this cat and still pocket the spread in the general case.

Sure, but doesn't that business model kind of exist then in the form of money market funds (and somewhat related in bank deposits) anyway? What's the new thing from a customer and business perspective?

Being able to take part in the defi ecosystem.

Being able to have all your money stolen with no recourse when you misplace your crypto pass phrase is new.