Give the flights a 128 bit number, so we can give each grain of sand on Earth its own flight number.
I'm pretty sure they want to keep flight numbers short because that's how passengers identify their flight. As a passenger, you wouldn't want to be reading signs with 40 digit flight numbers. Attention passengers, flight 3467548742468759027225995322800483168368 to Dallas is now boarding.
Maybe use QR-Codes to store/display the UUIDs, and everyone should walk around with Apple helmets (what are they called?) that scan these codes as you walk around in the airport... (Facetious idea)