If you trust the solvency of the likes of Tether and Luna, certainly.
Seems to me the problem of solvency at the boundary between the worlds of fiat and cryptocurrency is always present, and shuffling it around doesn't make it disappear.
But that's a choice for customers to make, at the end of the day. The exchange says "we're holding dollars in Tether and we can prove it" and if customers aren't happy with that, then they can not store fiat on the exchange.
Seems to me the problem of solvency at the boundary between the worlds of fiat and cryptocurrency is always present, and shuffling it around doesn't make it disappear.