I wouldn't mind MSQ completion of x.5 unlocking all currents in a given expansion, but I don't think unlocking all zones for flying is a good idea after completing the ultimate weapon. Just because you can fly in ARR doesn't mean you should be able to fly in Stormblood, for example, but if you complete 4.5 I'd be fine with them just giving you the currents you missed in Stormblood to catch you up.

The game does have a lot of momentum blockers, and flight is one of them when playing multiple expansions in a row.