Perhaps a better way to describe Smash Bros.'s identity is this rule: was this character most likely to be discovered through Nintendo culture? Sonic, while a rival to Nintendo, had a rivalry that came to characterize an entire era of Nintendo's history, and is now largely identified with Nintendo consoles. Any major Sonic fan is almost guaranteed to know a thing or two about Super Mario because their identities are so irrevocably intertwined. Cloud, while never appearing on a Nintendo console before, can claim a similar rivalry as an important mascot for one of Nintendo's largest competitors, but he is also the face of a franchise that had tremendous impact of its own on Nintendo's history, and helped define what it is today in a few ways. Bayonetta as a franchise was singularly transformed by Nintendo's involvement, and is now functioning as a Nintendo exclusive. Any Bayonetta fans coming to Smash Bros. are likely to have discovered this character through Nintendo culture in one fashion or another. Each of these characters compliment the focus of the game: Nintendo, its rich history and plethora of unique characters, and those stories that have come to define it, as well as those it helped to define. Through Smash Bros, we get to dive deep into Nintendo's rich history - who would have ever known about Mr. Game & Watch, R.O.B., Ice Climbers, Pit, or dare I say Fire Emblem, without this focus? While Goldeneye's near inclusion is a powerful thorn in the mind of Smash Bros. purists, it was only possible because of the legendary role that game had in defining Nintendo that era. Nintendo and Goldeneye are nearly synonymous in gaming because of each other.
Dragonball Z, while certainly bringing lots of people to Nintendo consoles, usually has people there because of Dragonball Z, not Nintendo. Dragonball Z is a franchise no Nintendo fan need ever learn about, and their knowledge of Nintendo heritage would not be hindered in the slightest. Likewise, no Dragonball Z fan need know much about Nintendo at all to be an avid Dragonball Z fan. They are disposable to each other - partnered only out of convenience, because they can promote cross-traffic. In other words, it is a quintessentially pragmatic, corporate partnership. There is no deep connection there beyond "my favorite anime has a game on this XBOX/PS/Nintendo console so I'm going to play one." Nothing that enriches the focal lens that defines what Smash Bros. is, which is Nintendo itself.
Remove that focus, and while Smash Bros. can be a fun game, it's stopped being about Nintendo. And with that loss, it stops focusing on the richness of Nintendo's own heritage, and the possibilities that can be explored and resurrected therein (Fire Emblem being the best example), and the focus becomes wild, broad to the point of diluted, and starkly lacking in identity. That this board is largely focused on resurrecting obscure Nintendo characters still (I never would have noticed fascinating characters like Lip, Isaac, Elma, or Bandana Dee next to heavyhitters like Spider-Man or Goku), is a strong indication Smash Bros is accomplishing exactly what it was supposed to: be a celebration, a time capsule, a gateway - for Nintendo itself. That is an identity to be proud of, and why I think Sakurai's hesitation against using fourth party characters is a very wise one.