So what you are basically saying is that if the dollar loses half of its value today, you can buy the Iphone for 0.02854BTC, then you could sell it again for 1600$. And buy 0.05708BTC for that money you earned from the Iphone sale.
Read my comment again! As I said, in that scenario bitcoin price in dollar would also go up and it would be be worth $56,000 meaning if you sell that iPhone for dollar and get $1600 you can still buy the same amount of bitcoin 0.02854
BTC not more.
But if I understand you correctly you are saying that as long as goods and services are priced in bitcoin, there wont be a problem with buying and selling in bitcoin, which is really a given - but how do we get to this point? And more interesting if that iphone is priced in bitcoin and it is also priced in dollar, euro, yuan etc. how will this affect eachother?
I'mt not saying that either. I'm saying that everything has a "value" and because of that value they gain an exchange rate when exchanges for something else. That something can be dollar, bitcoin, gold or groceries! As long as that value remains the same, the exchange rates remain the same too. But if value of one of these (like value of dollar) changes the exchange rate of everything (iPhone, bitcoin, gold, etc.) against dollar would change.