This. It's made very clear that evokers are the minority when it comes to Dracthyr, and that they're the best of the best that the Dracthyr have to offer. Some NPCs even mention that they were unable to complete evoker training and are jealous of the player. I think it's cool that they can only be evokers, but I really think that's just a temporary thing for Dragonflight, and that they'll be able to be other classes in future expansions.