I can see that, but, personally, I do not care about those things. I think game design trumps. If it is more fun and/or makes the job of the GM easier, I'm for it.Sure, but I'm not talking about that. I'm talking about the idea that an NPC Archmage has an "arcane blast" that a PC can't learn because it's presented as a monster feature in the statblock, not a spell.
You think verisimilitude is of the highest importance -- a view that I respect, but do not share.I believe they should potentially have that access if the NPC is the same kind of creature.
That said, there really isn't a game design reason for Arcane Blast, when choosing a cantrip and putting it in the stat block works just as well.