For example people could keep their savings in bitcoins and avert losing them due to inflation, defaults or taxes.
Meaning no one will ever buy anything.
defaults
Are insured by the government.
Not so with bitcoin, where the various defaults we have already seen always and invariably end in total loss. Total loss for the investor, mind you. Not the owner. The owners are in the Bahamas sipping cocktails.
taxes.
Keep your house from burning down. We've seen what happens with privately financed services. Your house burns down.
Transfer fees
Try to transfer bitcoin without a fee. Protip: Don't try to transfer bitcoin without a fee.
exchanges between different currencies,
I'll give you that
capital controls
Lack of capital controls after Bretton-Woods ended is exactly which led to the Latin American debt crisis, the East Asian Financial Crisis , the Russian Ruble crisis, and the Global Financial Crisis