I should have been more specific. I think Decidueye isn't the most obvious character because there's like 5 more pokemon that are equally valid as him. Spring Man is more of the timing issue, so I lumped both into that.
Also, I'd say Bandana Dee is more obvious than Decidueye and Spring Man at least, it might be bias that makes you think otherwise, lol
While I agree there are Pokémon that are as valid as Decidueye, generally he is the most popular Gen 7 choice and is considered the 'shoo-in' Gen 7 Pokémon for the Smash fandom as a whole.
It doesn't really matter if there's five other options if said five options are relatively ignored in Smash speculation, and whenever it comes to 'top 10 most likely characters list' or 'roster predictions', he tends to be the one that people pick. He's about as obvious of a Pokémon choice as you can get from the franchise at this point.
As for ARMs, while the belief of it being too late to get in is starting to spread, it's still not widely held information and it's not uncommon for fans to consider Spring-Man a shoo-in due to the status that ARMs carries as a new popular IP. Hell, even in places where that belief is held, a lot of people still feel ARMs will be the exception due to that status and what happened with Greninja.
If someone was trying to make a fake leak to get 15 minutes of fame, both of them would be valid choices to get attention. You would get a lot of fans talking if you put in Decidueye and Spring-Man over say, Dillon.