I'm not arguing fiat currency will disappear, it obviously won't. It's just not the correct vehicle for saving. You must not save in the same instrument of the debtors. It leads to you being wiped out when the debtors default as the credit system grows beyond the ability for it to be paid back (eg: see China's trillions of US Treasuries - those are claims that the US economy cannot and will not service).
re: Spain: It was silver that was debased not gold I think.
re: gold failed: Physical gold didn't fail. Gold backed currency failed. Governments always print more tickets to gold than they own. The last big paper gold failure was Nixon in 1974 after they overspent on the Vietnam war.
I wouldn't recommend getting into paper gold to escape a currency collapse. Physical, unambiguously owned metal is the insurance, the vehicle that will transition you through to the otherside.