Can you raise the base speed of several mounts, or reduce some?
Stormdarter and Cloudstrike Panther comes to my mind. They are bot at 10.0 or 10.5 (not sure), and they both need a bit of investment. However, nearly all the rng box mount is 11.0 or 11.5 these days, and I don't see why it is. They absolutely devaluate the previous ones, which you worked for, and not just bought for several thousand gold. And these rng box mounts most of the time even have superior abilities, so there is no reason to use other mounts over them.
So I guess you won't reduce the base speed of the rng mounts, then my suggestion is, to rework the ingame mounts base speed, so they match with others (even a 11.0 would be good), and not completely obsolete (in this game, it really hurts when these cool things become obsolete, especially because of the rng boxes, so please consider it).