Isn't this the exact idea of these conversions -

No, the idea of these conversions is to have the right amount of HBD printed. It has nothing to do with stabilising HIVE price.

If the price recovers in the next 3.5 days you'll lose half of your Hive in the conversion

It is the other way around, You buy HIVE off an exchange, convert HIVE->HBD and get HBD cash. If HIVEUSD recovers, the conversion result is that you sold HIVE for the recovered price so you get almost all of your colateral back (making huge profit on the rest as you got disproportionate amount of HBD 3.5 days ago).

But it is not "at no risk".

The actual risk is that if the price fails to recover, HBD depegs. Not only old HBD loses a huge amount of backing (debt ratio shoots up) but you and your peers printed lots of new HBD to make things worse. In fact market is going to expect this situation so in practice, HBD does not wait for the conversions to settle, it depegs instantly. No time to dump the newly converted HBD even though you get it on the spot. If your gamble on HIVEUSD recovering fails, you are getting your "profits" in USTerra.