slobster
Hero
It is certainly a valid choice for a DM to say that because a nonplayer character's role in the story is different, a different mechanical structure may be used to define the character (perhaps simplifying or altering the mechanics to produce the blacksmith of his dreams), but that's not a choice the designers should really be forcing on everyone and all their characters.
Yet they can't stop you. If they make character classes for use by players (pretty much a given for 5E) then they are simultaneously giving the GM everything that he needs to populate his entire world using the same rules. So I wouldn't worry too much on that account.