The moment all the countries convert to crytocurrency would be the beginning of many amazing things to happen to the world and the financial sector, the process sending and receiving funds worldwide would become very fast and easy, it would lead to growth of businesses and make life easier for citizens of the world.
you say it right but if all countries convert to bitcoin then bitcoin delivery and receipt will be very slow, you can imagine yourself if every day there are 1 million more transactions while 1 transaction takes 15 minutes, so you can imagine it will be very slow with this long queue, and this is specifically for converting to bitcoin, but it might be different if you convert to another coin that has a faster confirmation speed than bitcoin.