I think regardless off the current situation that Geno is inevitable. There's always some reason the next never ever character can't get in but they always do.
Ridley: "Too big and Sakurai already vetoed him."
K Rool: "Sure, Ridley got in, but K Rool is irrelevant and forgotten."
B&K: Well yeah, K Rool got in, but Banjo is irrelevant, forgotten, AND third party."
Geno: "Banjo may have gotten in, but Geno is irrelevant, forgotten, third party, AND not a main character."
Each time "it's different because of X" and each time Sakurai adds the character anyway and cites fan demand as the reason. Despite the "smash bubble" argument, the "promotion" argument, and the "he's not popular in Japan" argument, the never ever characters are getting in one after the other and Geno is the last one left.
His time will come sooner or later.
Which is hilarious, cause some of these are completely incorrect. B&K are absolutely relevant by their franchise being treated as active. The constant merchandise, the advertisement, and very consistent re-releases via Rare Replay. Never mind the opposite of forgotten(that doesn't mean he isn't a niche character. He's iconic, but on a lower scale. Not that that's a bad thing).
Geno was always a main character. Which makes that such a bad argument. He's part of the main story and always was. Peach is the only one that really isn't important to the overall plot, and gets kidnapped twice, but both of them do lead to some team-ups and such. It also was a fun way to reenact the class Super Mario plot. The second time was a fun way to show off Bowser and Mario being silly but also showed they cared about her. Mind you, it still suffers from the "object to be saved" issue in itself, but that's... unfortunately going to stay the same in Super Mario for a longggggg time. Though hey, at least Odyssey had her tell both off for acting ridiculous?
Rest, yeah, correct. Mind you, what is incorrect is it's not inherently about the character alone. The franchise matters too. King K. Rool came from an active franchise, so it was easier to get him in over other fan demanded characters. There's a fair chance he'd get used again at some point, due to being the series' main villain. That's something many characters don't have a chance of to any near degree. Not even Wolf had a good chance of that due to Star Fox being dead for a while, and then typically Star Fox Zero came out way too late for being a good way to bring back Wolf with promotional(meaning he's active again) excuse, though part of it would still be veteran. To be clear, SFZ's delay meant Wolf lost relevancy again, so he missed his chance. Geno's biggest thing is that the Mario series is always relevant, which is a pretty good reason for Sakurai to consider him a great choice as playable, though we don't know why he hasn't gotten in yet. Many possible reasons. I won't go into them, mind you. That's not worth rehashing. He acknowledges the character should be playable. That is nice.