This is a noob question. But I couldn't be sure if I understand it right.

For example, we have 4 million BTS in the first chain and suppose each BTS is worth $10 in the beginning. Let's assume that 50% of the BTS are converted into BitUSD at that time, then we would have 10 million BitUSDs in BitShare BEX.

If later the price of BTS dropped to $1 externally (not necessarily a sudden drop, it can be over months or even years), and BitUSD would rise also against BTS. Is it possible all those 10 million BitUSD would still exists in BEX?

My understanding is if no one covers their short positions, those 10 million BitUSD will be destroyed by forced margin call as soon as BTS drop more than 25% of its value against BitUSD (BTS that's backing the BitUSD will be returned), if some one covers their short positions, then at least some of the BitUSD will be destroyed and there will be less than 10 million BitUSD available. Therefore total amount of BitUSDs are limited by the exchange rate of BTS/USD? Is that right?

And even if anyone is able to predict that BTS price will drop from $10 to $1, there is no way he/she can act on it to preserve the dollar value within BitShare BEX system, e.g. converting BTS into BitUSD doesn't guarantee that you will own BitUSD for as long as you want, as the BitUSD will be converted back to BTS as soon as the BTS backing BitUSD falls more than 25% against BitUSD.

Am I missing anything or the BitShare BEX cannot protect the value of BitAsserts against a 25% price drop of BTS?