The accusative case in guteN indeed comes from wishing it for the other (English is grammatically not expressive enough here).
This is the same BTW in Hungarian. Even better, in HU we still have the form of "I wish (you) a good{Acc} morning", however a bit less formal way you can say just "a good{Acc} morning"
This simplification happened to German earlier too. In fact, my German old lady neighbor still to this day says every time: "Ich wünsche Ihnen einen guten Morgen / schönen Tag, Herr kmarc"