To me the problem with non-game characters in Smash isn't "losing the focus" but rather that it'd give Nintendo a bigger monopoly over the platform fighter genre than they already have.
One of the main reasons people were hyped about Nick Brawl was because characters like SpongeBob, Aang, the Turtles, Ren & Stimpy, e.t.c. are popular enough (if not more so) to draw as much attention as Nintendo's cast of characters - that would've been lost if Goku got in Smash beforehand and SpongeBob was on the table.
One of the main reasons people were hyped about Nick Brawl was because characters like SpongeBob, Aang, the Turtles, Ren & Stimpy, e.t.c. are popular enough (if not more so) to draw as much attention as Nintendo's cast of characters - that would've been lost if Goku got in Smash beforehand and SpongeBob was on the table.