It'd be a little cheap of them to not include it in the base game if it's handled the same way as Breath of the Wild's was since it's obvious that it would come later if not and doesn't require much changes to upscale enemies. But I suppose even if the standard is set, there is a possibility they'll make the difficulty less artificial with just enemy upscaling, so I'll forgive it coming later if they have a bit more changes to master mode than what we got with Breath of the Wild and it might require more development time, if so.
But yeah if its identical to how BotW's is handled, there's no reason it shouldn't come packaged with the base game.