It's not like it didn't happen before (it happened twice), and their rate diverged from the 1:1. Actually, their rate wasn't that super stable until very recently. The reason why I think they are fully backed is that the crypto-market yield for USD in the last few years have been crazy. So unless you are planning an exit scam, you can't go broke. Even if they are losing money to move the money, or they screw up here or there (get accounts blocked), they can still come on top.