I agree op. The cost should be every time someone uses a flying mount in drains game time from someone who posts about how much they don't like them. That people who have a problem with something that a vast majority of other people enjoy, can run out of game time faster and stop playing, therefore stop coming up with ridiculous rubbish suggestions like this:
It's not an incentive for using ground mounts, it's a pointless penalty for using flying mounts because you and your pathetic gank squad can't harass players 30 levels lower than you endlessly.That would make flying mounts to be there just to be used as a last resorts for many and create incentives to use ground mounts more often.