I like the concept. Like Ion said: you want the armor set? Earn it!
Ofc Blizzard charges you if you wanna Race-Change because you don´t want to level another toon... why the hell they shouldn´t ?
They hadn´t give away free race changes back in Cata where they introduced more Class/Race combos... why should they now?
IMO the only thing which is discussable is the fact that the only way to "unlock" the armor sets is via leveling, maybe there is a way to make it possible for players who pay for the race change to achieve the sets via Max-Level content. So they still would "earn it".
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My plan is leveling a lightforged draenei Priest for Battle-Priest style