You are right, even if Goku was just a spirit, and not an assist trophy nor playable, I would still be happy.
But assist trophies aren't playable, and they don't join the battle. Imagine how amazing it would be to see Goku appear as an assist trophy in duel between Mario and Sonic. Since assist trophies don't join the battle, it justifies why Goku can only be an assist trophy or only a spirit, but never playable. Even if fans continued to support Goku, in Smash 7, after Smash 6, Sakurai would most likely remove or retain him as an assist trophy.
Also, would having Goku as an assist trophy in Smash 6 lead to having more anime/manga assist trophies?
In fact, Goku and Mario actually had a minor crossover in a SNES commercial prior to the release of Smash 64.