The flying mounts will be released alongside new zones, which will no doubt be designed with flying mounts in mind. Even if there are zone lines that doesn't mean what's inside each zone won't be fun to explore while flying.
Just because some things aren't as useful at higher levels doesn't mean they should be removed from the game. We "outgrow" many things like quest hubs, chocobo porters, guildhests, and hundreds of pieces of gear...but the new players will still want these things and there are some veterans who enjoy airships (I have one in my FC, she uses airships to travel from city to city almost exclusively instead of teleporting) and features like them.What is the point of having airships like lvl 15-17 story quests when you just teleport. Sure might be tiny pricey starting out, but around lvl 30, no longer useful. You make enough money you can just teleport.