Hive's haircut rule is much more robust than similar mechanisms on Terra. It was possible for Terra to print trillions of LUNA for people trying to convert UST. The same would not be possible with Hive. Terra did have a speed bump in how it does conversions a little bit similar to the haircut rule, but for some reason in the last few days they decided to essentially remove it. I guess it was a hail mary to restore the peg, but it turned out instead to accelerate the crash to zero.

We'll never know, but it isn't impossible that if they had left things alone with the speed bump in place, it might have recovered (I'd say unlikely, but not impossible). If nothing else it would have slowed down the collapse and allowed more time to implement a better fix or negotiate with investors for a work out (which was tried but the negotiations didn't succeed as things played out, perhaps because the situation was so rapidly deteriorating).