At the end of the day, it's a cosmetic, it's not Pay to Win, it's entirely optional.
And while this particular design could have been implemented in a different way, something would have had to take it's place.
The cash shop is part of their business model, they will be releasing a new mount in the mogstation every few months, it's already scheduled, best get used to it. The only thing that's going to stop it, is if it stops being profitable.
And sure, I get it. "why does the specific mount I've always wanted have to be the one that gets put in the cash shop?" Well it's a popular design, and they're going to put stuff in the cash shop that will actually sell.