Anyway, I don't think the fee is the way to go.
I don't know what problem it is trying to solve by weakening the lower bound. In the case of UST, even if there was a 25% fee, it wouldn't matter. I would argue it would even make it worse. Without the fee at least there's a promise of $1 value that keeps the panic farther away.
We have $1 value promise as long as the haircut rule is not in effect and I would say that gives enough confidence in the HBD to not cause any sort of panic. Even if panic happens, we assume the HIVE market can absorb the debt because the debt is not that huge.
I think you could argue 25% fee is protection against the HIVE price going down. So we let HBD go down to not print more HIVE. But I don't think it's worth the trade-off.

Edit: The haircut rule is doing exactly that though. It keeps printing less and less HIVE per HBD when it's in effect. So you could say we already have this mechanism.