OK in my example above, to profit by selling Nubits is when BTC is expected to rise, not to fall. Park rate is offered so the trader gives up on speculative selling for park interest. Although a rising BTC (if it really rises as expected!) does make mopping up the printed NBT (interest paid) easier, using park rates to defend the buy wall seems still a questionable thing to do.
Yes there are several factors if a more sophisticated model is to be made. But the idea is the same. Shareholders have to figure out the right rates in a dynamic environment. That is very hard speculation.
The way out, as I see, is sticking to organic growth of Nu, only maintaining the NBT/USD peg.