My first impression is this is totally unnecessary. Bob knows he has to buy back that dollar sometime, and there will be a lot of Bobs. Alice knows if bob runs out of collateral he is forced to buy it back at fair price.
It also gives too much power to the price feed. I wouldn't use the price feed for anything other than eliminating new shorts below the feed price.
I think the idea is that in Alice's mind she would be absolutely secure that she can sell at 90%, at all times. Just like the system bot proposal but without a bot.
That's nice an all for Alice. But Bob is screwed, going short is all about risk reward, and now we're cutting the reward for bob.
I agree. Suppose the market has equal chances to go in both directions (you don't really assume that it will always grow, do you?
. Bob bets on BTSX, Alice bets on BitUSD. If BTSX goes down, nobody is forcing Alice to sell her BitUSD, so she gets more profit on average.
Also, don't forget about Bob, he can get into the same situation, suppose he wants to exit his short immediately, but there are no BitUSD sellers (or other shorters).
So it looks like a double punishment for shorters. The imminent consequence of this is fewer people willing to short, and as a result slower growth of BitUSD supply.