> Plus any solution has to be approved for use worldwide.
I don't see why for instance you could do a slow roll out by using only letters for internal flights of countries that are ready for the switch. If you did that at first only for the USA that would probably leave enough room for Delta and the 2 other companies that need them in the short term. And then you upgrade the rest of north america: Mexico + Canada, then you continue the roll out. I am pretty sure all european countries could coordinate to do the switch at a specific date in the future as well.
I thought of this but you need to be able to book flights in Europe from, say, Mozambique. Flights from Europe may overfly Kazakhstan ATC. And so on. It’s a system.
I don't see why for instance you could do a slow roll out by using only letters for internal flights of countries that are ready for the switch. If you did that at first only for the USA that would probably leave enough room for Delta and the 2 other companies that need them in the short term. And then you upgrade the rest of north america: Mexico + Canada, then you continue the roll out. I am pretty sure all european countries could coordinate to do the switch at a specific date in the future as well.