Talking coins here. They never go out of circulation. Can last decades, a century even. And a 'grace period' for coins is what I was getting at - they get stuck places, left for years. Billions will become worthless if there's a cutoff.
And further, coins are exactly that currency that's supposed to be intrinsically valuable! The founding myth was, they're precious metal and that's why they're tradable. A cut-off defies that myth. Unsettling.
This is hardly without precedent - the old 50P was demonetised as recently as 1998. The world appears to have coped.
(And "Billions" - Wikipedia claims there's an estimated ~1.5 billion pound coins in total circulation. The overwhelming majority of those are going to be exchanged during the changeover period, and even after that banks will tend to accept them for a significant amount of time. There's no way that the amount of money written off is going to be anywhere near that)
That’s not true in the UK. The farthing was removed from currency in 1961. When we decimalised in February 1971 the old pre-decimalisation coins went out of circulation in August 1971, only 6 months later. Even more recently the 5p and 10p coins changed sizes in the early 1990s and the bigger ones went out of circulation a year later.
Obviously you can sell coins even if they’re not commonly accepted by traders (you can buy trivially buy roman coins), but good luck trying to use one in a shop.
And further, coins are exactly that currency that's supposed to be intrinsically valuable! The founding myth was, they're precious metal and that's why they're tradable. A cut-off defies that myth. Unsettling.