Once everybody only uses electronic forms of money, nobody will be accustomed to use physical coins and regard them as valuable. In addition there will be no new coin issues, marking the end of an entire collecting field. The probability of new collectors entering the numismatic scene will be greatly reduced as a result. Once the old collectors have died, there will be no new demand for coins.
Apart from that, true collectors don't collect because of a potential increase in value of their collection.
ya.ya.yo!
Ok, Well if this was true... then explain stamp collectors. We hardly use any stamps anymore, since most people use email and social media.
There are still millions of people around the world collecting stamps. So, I beg to differ on your... "beg to differ" ... I think even more
people will get into coin collection, if Bitcoin or Crypto currencies replace Fiat.
Of course there are still collectors of stamps. I did not say that the number of collectors will suddenly drop to zero. But there will be significantly fewer collectors and hence overall lower prices (inflation adjusted).
In fact, the collector values of most common stamps are trending downwards since more than a decade. The only exception being very rare stamps in the highest price segments which have attained investor demand as a portfolio diversification. The same will happen with coins: Only the rarest and historically important coins will preserve/increase their value. Ordinary precious metal coins will keep at least their material value of course. Ordinary non-precious coins or coins mass-produced only for collecting purposes will see a sharp decline in value.
ya.ya.yo!