A collapse of the dollar is extraordinarily unlikely, and should one happen your BTC will be of little use to you. Such an occurrence would cause parallel collapses in the supply chains that maintain the web, the energy production and distribution necessary to keep running all the PCs and servers on which BTC exists, not to mention the supply chains that provide everything you would want to buy with any other form of currency, which BTC definitely isn't. The designed in scarcity guarantees that BTC can't, and never will be used as a currency, leaving its one function as an electronic bank with the defining characteristic that funds deposited are held in a volatile form what will become ever more difficult to exchange as the blockchain grows.