This is an interesting point, but I think you're putting a bit too much stock in that Nintendo
has to cater to kids, and that is why Steve has a realistic shot of being in, in your mind. Presumably kids are going to already be interested because of all the classic Nintendo characters and the addition of Inkling, and maybe whatever new Pokemon gets added to the roster. A big part of the Switch's marketing so far has been geared towards adults in their 20s and 30s, which makes sense, because they're the ones with disposable income, and they're the majority demographic for it.
Meanwhile, and I know this will sound very condescending, kids' tastes are more fickle, while there are people who would have straight up not bought this game - hell, even bought a Switch - if K. Rool and Ridley were not in it. Most everything in this game has been geared towards longtime fans (especially those with mixed or outright negative feelings towards Smash 4), and most of those kids requesting Steve would play this game anyway. On the other hand, the backlash from this older community if Steve was playable instead of Banjo would be unfathomable.
Also, characters like Dark Samus and Chrom don't exactly have to do amazing on the ballot to get in. Just decently enough to be turned into a quick clone for fanservice. As an example, Chrom outright won a Tumblr poll, beating out even K. Rool.
Also, it's been established that Sakurai is a fan of Castlevania, but it actually isn't too farfetched that Simon did better on the ballot than previously thought.
On a compilation chart of numerous polls, Simon was ranked in the top 20.
Ultimately, I'm not sure why it has to be "Steve should get in over Banjo so kids can have their fan favorite." If you think Steve should really be in, the case should be made that he can exist alongside the (hypothetical) roster, not displacing a fan favorite among Smash's target demographic.
To start, your point about Castlevania can actually help my argument as Sakurai is actually a fan of Minecraft saying that it’s one of the few titles he actually returns to.
https://sourcegaming.info/2018/02/2...mple-actions-sakurais-famitsu-column-vol-535/
Plus, you also argue that kids or primarily casuals (Not all MC fans were kids, some were probably just regular people who didn’t follow Nintendo) would buy the game regardless, but doesn’t that work more for the general Smash community? If I’m not mistaken, many Smash fans have already said that they’ll buy the game regardless of what the Newcomers are. Even before the game got revealed, many said it could just be a straight port of Wii U and they’ll still purchase it. While I think pleasing fans is important, now that we K Rool, Simon, Ridley, Inklings, and Chrom, why shouldn’t the team include a more casual character that would be widely recognizable to today’s youth, encouraging them to pick up the game when they see him on the box. MC has millions of fans, so including Steve would be a good decision to draw in a new crowd for Smash.
My final point would be that there is a bigger picture to why 3rd parties are chosen, as I believe that the character needs to mean something. This of course varies as obviously bigger genres like Platformers are hard to find franchises that impacted it, while the Hack and Slash genre has like two faces in total, but you get the point. These characters either needed to pioneer something in their genre, perfect their genre, or be a gaming history character or a character that symbolizes a period in the gaming industry. To go through all;
Snake; A member of the action/stealth genre, while Metal Gear is basically the face of stealth games, Metal Gear is more recognized for really pushing story in games as many see Metal Gear’s story as the best video game take ever written, especially with 4. Metal Gear had a huge hand in making games more cinematic than ever and basically helped pioneer the new trend in action titles like the Last of Us where the focus is very heavy on story.
Sonic; A member of the platforming genre, Sonic greatly influenced 2D platforming as Sonic was more free than a Mario, having loops and hills, and very ‘organic’ level design. Sonic also represents the first ever console wars between Nintendo and Sega, giving him an edge in the history department.
Mega Man; He perfected the NES 2D platformer. Many will talk about how Mega Man perfected the genre and was a symbol for the NES with great level design and challenge, especially in the case of Mega Man 2.
Pac-Man; He is literally the face of arcade gaming, with many creators looking up to Pac-Man as an inspiration, such as Miyamoto. Pac-Man is a historical character through and though, so of course he had to be included.
Ryu; Pioneer and face of fighting games. Street Fighter, and primarily its sequel have become somewhat of the peak of fighting games with the only really notable competition being Mortal Kombat, so Ryu deserved his spot.
Cloud; Final Fantasy is the most main stream RPG out there and many follow in its foot steps, so it is a huge pioneer in the genre. With FF7 being the biggest game of the series, it was only natural to include Cloud.
Bayonetta; Probably the hardest to justify, but I would say Bayonetta is the perfectionist of the hack and slash genre. When it came to representing this genre, there were really only two choices, Dante and Bayonetta, so it really shouldn’t have mattered much, but most YouTube gaming channels, specifically the ones like Game Makers Tool Kit have pointed to Bayonetta as the front runner of the genre, as she took previously established mechanics and perfected them in a way while adding her own style. While she is niche, she earned her place for being the best in a popular but relatively small genre.
Going over that, I believe that Sakurai looks for 3rd parties based on something bigger than just popularity. The character needs to be the best, or the thing the best have taken from for years. 3rd parties are special as its a privilege to get in, or something you have to really earn, which is why I see Minecraft as there better pick. Minecraft is not only popular, but it’s a game that took the relatively niche genre of world building/ survival games and brought it into main stream popularity. It helped simplify a genre that was too complex for most to grasp, and made it accessible to even a 4 year old. MC is also really the gamble in its genre to perfect the gameplay, many try to emulate it but they make it too complex or boring, leaving MC to be the perfectionist of the genre.
If Microsoft is to get a rep, then I personally think Steve would be the choice as he is a gaming great up there with the rest. I also think he’s good because he represents a genre that isn’t the platformer, a genre that already has 4 members already, and if you include the big man himself, Mario, then that’s 5 platforming giants right there. The World Builder genre literally has no representation in Smash, and with Sakurai’s goal of representing gaming as a whole, wouldn’t it make more sense to have Steve as he takes on a genre not represented yet?
With one 3rd party slot left open on the base roster I think Steve should take it over Banjo, we need more genres to pop up and diversify the roster. I think Sakurai is going to slow down a little bit on platforming giants this time around to complete his goal, though I can see one more joining as DLC, though I think Rayman might actually beat out Banjo to that title as Rayman’s 3D platformers had a dark story added, something really only seen in Sony’s titles, he’s the only one besides DK to actually advance the 2D platformer in the modern era as most are just doing retro throwbacks to the NES, and the Rabbids titles that spawned from his series were some of the biggest party titles ever, rivaling the likes of Mario Party. (Which yes it still counts as he was in the first few titles, and really the downfall of the Rabbids leading to the Mario crossover was after Rayman departed) I don’t think Banjo is out of the realm of possibilities, but I feel that there are characters that are just more important than him.