Banjo was a highly requested character since the very beginning of Smash. His games were N64 classics, and people were wondering even at the release of Smash 64 why Banjo wasn't in, at least all throughout America. That's a long history of fan demand, and to this day, it's a beloved franchise. Rare had a lengthy history with Nintendo, and it was just pure awesomeness that they agreed to lend the character, Yes Rare is owned by Microsoft, and obviously they were also involved in the decision, but I really think people need to move beyond Microsoft and realize it was probably mostly through Rare that this happened. And these types of relationships help build stronger collaborations down the line for sure. And Banjo games probably got a boost in sales when he was announced, through the Rare collections and whatnot on XBox.
Now Geno... Geno has a fanbase, but where does it come from? We know for sure that Sakurai and other developpers hated the whole Waluigi stunts and spam, and Geno is kind of in the same boat. He's popular, but from what, the memes? Because whenever you ask people in the Geno fanbase why it's him and not Mellow, they don't really reply. Was Geno an actual fan favorite back in the day, before Smash, during SMRPG? Do you think Sakurai will try to please the fans? If Geno gets in, why not Waluigi? (or vice versa?) And then again, get out of the Smash Boards bubble, and would the rest of the community actually care for Geno? Is it worth the tricky investment into the character due to the current ownership situation, when it wouldn't necessarily appeal to the casual community at all? A community that forms what, 70% of the playerbase if not more? At least characters like Min Min, Hero, Banjo, they appeal to those quite a bit more, through history, design, originality, look and whatnot. Compared to a character that has been in only 1 game ever?
So yeah, to me, comparing Geno and Banjo is like comparing apples and oranges: they're both fruits demanded by some fans, but beyond that, they have very different contexts, and while a respectable pick in itself, I find myself struggling to see Geno with such big odds as some would claim here.