I'm not so much Smash speculating more than just confused about the fact people don't think Splatoon will be an ongoing franchise considering it's success and merchandising thus far.
Regarding Corrin, I'm not speculating him, he's in the game already. What I am debating though is this notion of people trying justify his inclusion through sales and popularity his game does not have.
I get it, FE fans have seen a lot of heat this iteration due to them getting so much in terms of characters and yes, one was a clone and one was a former clone that is still relatively similar to Marth. But if they want to defend Corrin, just point to his moveset.
But trying to use sales and popularity? No, they are arguments that don't apply to Corrin.
Corrin was added to the game in hopes that Smash might boost the sales and popularity of his game, not the other way around. This was flat out stated in the direct, it is objective fact and it cannot be argued. So I don't get why people even brought up past sales as a possible reason for his inclusion.
"We hope you will enjoy getting to know Corrin in the lead up to his upcoming debut in Fire Emblem: Fates"
-Masahiro Sakurai
*may not be the exact quote but he pretty much said that
I posted TLoU as an extreme example but since you are still missing my point:
The Game Awards are a huge event that a lot of people tune in for (I mean tons of people in this very thread tuned in for it even), and people genuinely do use the awards as a reference for what games are good. Furthermore, any sort of noteworthy award is "box art material".
Splatoon beating juggernauts like Call of Duty, in two categories that are generally won by those juggernauts all of the time, is a very, very, very, very big deal. It puts it on the map, people have reason to believe it's a better game than Call of Duty in more than one aspect.
Also Splatoon does not need a sequel within a year of the beginning of Smash 5 development.