I didn't read all 4 pages of comments, so this may have been stated already. First, that is a creative and well-thought out idea that deserves proper consideration.
But it fails to address the issue. It sounds like this is a mechanism that would prevent people from using flying mounts like the taxi flypoints. For example, if I were out flying from digsite to digsite working on my archaeology, I need to fly from zone to zone for extended lengths of time, and unless it's in Pandaria, it's all over trivial content. This is the kind of thing that the mounts should be there to enable.
What Blizzard has stated they are against is people using the flying mounts to avoid danger in the world from NPCs and other players by flying over and out of range. The mounts can be used to cheese certain mechanics while farming herbs/ore as well. This is what they want to fix: certain short-duration uses, and unfortunately I can't see how your suggestion might address these.
If you were to suggest a limit, that limit might be better focused on the number of times one can mount per unit time forcing you to stay grounded and fight if you had used up your opportunities. I think ultimately such a limit would be hard to work into the game because it would just become a QoL issue people would complain about.
Blizzard's solution: rendering new zones unflyable seems to do a better job of addressing the problem by establishing the ground rules (pun intended =) from the beginning and designing around that. While it may seem like an inconvenience, they're thinking of where roads/high traffic areas should be, mob density, respawn rates, where mineral nodes/herbs should be, etc and keeping things reasonably enjoyable. I won't say they're always perfect, but I think they're aware of a lot of the potential unintended consequences of ground only rules and are working accordingly.