I think the complete opposite would happen if Bitcoin replaced fiat which won't happen anyways as already noted above. Because when fiat is replaced by digital money the physical money would become obsolete and thus worthless. Their value from a collector's point of view will plummet because there would be no interest in keeping such outdated artifacts any more.
banknotes and legal tender coins (nickels dimes, cents, 20p pieces and pennies) will be useless..
but asset collectables (1ounce gold coins) will have a place.
it wont be a big place. but will see growth of utility. just like bitcoin.
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again not everyone will do it. and bitcoin/gold wont replace fiat. but it will become a open choice to swap into
I would agree with your comment about 1 oz coins.
But, I would *speculate* that truly rare numismatic coins, especially (graded in all cases by NGC or PCGS) rare gold or ancient coins will do very well even if Bitcoin gets dominant (if ever). There will always be collectors of the very highest quality, whether art, antiques or top real estate (oceanfront homes for example).