What do you think is the likelihood of a 2008 financial crisis happening again if we only used bitcoin and there were no fiat money at all?
First you need to understand why the subprime crisis happened in 2008. It has nothing to do with the monetary system or currency issues. It happened due to the flawed practices of mortgage lenders who kept on lowering lending rates and selling out derivatives to securitize the lending. Do you find any relation between the currency system and the reason of crisis?
So having bitcoin as a currency without any fiat wouldn't have changed the fate. No matter what currency system we have, this had to happen!