Again, this is the WORST argument of them all. To think that Blizzard would be okay with any amount of cancelled subscriptions from something they change is kinda dumb. Yes, i can concede that they probably have a number that would be considered an "acceptable" loss, but even with that no company is going to be willing to do something that they know would cost them money. Removing flight (even just in one portion of the game) would cause less people to be inclined to buy new store mounts, even if they reduced the cost of those mounts if they only had a ground option, and to think that people would be willing to pay the same amount for a mount they only get half use out of for the majority of the content they will be participating in with a tagline of "Don't worry, you can use the flying aspect of this new fancy mount when you visit the old content!!".
Also, if you think cancelling a subscription is "childish" you're ignoring the very real fact that actions speak louder than words and by cancelling they show their opinion very strongly to Blizzard about flying being removed. Blizzard doesn't care about the maturity level of the player base so long as said players are paying their sub fees. That's clearly evident by the conversations in Trade Chat on nearly every server.
- - - Updated - - -
Person was referring to the one and only flight path that actually goes to Timeless Isle. Yes, many will connect to it, but it's in the middle of Jade Forest and it's a long flight from there to Timeless Isle. I think we've all been on many flight paths that don't go directly to our destination as if we were carrying multiple passengers and there were layovers along the way, many of those "stops" are out of the way and cause you to circle around your destination just to get there.