The argument that instant is necessarily because walking takes too much time is not really valid. Walking takes 20 mins a normal airship ride would take 3-5 mins. That is already a massive short cut. There is not need to bring it down to instant because you already have anima for that.

Chocobos will reduce anima use by eliminating the need to port to any camp level 30 and under.
A regular airship would reduce anima use by eliminating the need to port from town to town.
Return already allows you to get to the place you use most without using anima

Those three things alone will cut your anima use in half. There is no need to make airships instant because you will have the anima to spare from now on if you desperately need to get to a town instantly.

I would also like to add that the airships in FFXI only took so long because you actually had to wait for them to arrive and take off. That problem is eliminated by simply making airship constantly run rather than once every 12 minutes.