You're not wrong, but my gut tells me there is more mass appeal from a DLC character from Yokai Watch. Layton is popular, but he's definitely way more niche with the general public.
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I think the problem is that Jibyanian is NOWHERE as popular as he was in Japan back in the mid 2010s. The frachise's popularity was mostly just a giant fad, not some next big mainstream icon, even they have moved on from Yokai Watch. I mean yeah, the series still exists, but it's now just kinda there. and Yokai Watch never got big or even moderatedly popular in the west (mostly due to the franchise reliying heavily on Japanese Mythology and folklore that the most western audiences simply will not understand) and seeing how long it took us to get JUST the Hero from Dragon Quest......yeah, not good for him.
His best chances were in the base roster of Ultimate, when Yokai Watch still had some popularity, but now that time has passed. Just as you said in the last page.
Compare that to Professor Layton, who is more well known worldwide (we get the more internationally recognizable FF before DQ, remember that) has seniority on his side (one of the reasons Jibanyan was not even considered for Smash 4 DLC was because he was just too new) is much more requested by the Smash community, etc, etc ,etc.
Yes, Layton is not exactly relevant anymore, but neither was Banjo, and he got in Smash before the main player character from the best selling game in the planet. We have gotten "niche" characters in Smash before, Mega Man over Resident Evil, Simon before PES or Power Pros, Banjo before Steve, Bayo before Arle and Kiryu, etc.