my point was more so that if you're creative enough, nothing poses an impassable obstacle when it comes to development. Introduce areas where mounts are ineffective, or limit the capacity of flying mounts in their effectiveness. How about foggy areas that they can't navigate? Areas of rocky terrain that they cannot pass over, or even just revoke some of their effectiveness, possibly enabling flight over water but at reduced speed?

I could go on with ways to limit the ability of the flying mounts in particular, but it remains that this is simply a bottleneck with which to encourage community members to cough up a few bucks to get a (previously prestigious) ability which has outlived its novelty due to it being given out at cost. It may be simplest to introduce something different and leave the mounts as they are... the options are limited only by imagination.