Except we already know the Pokemon Company has a vested interest in Decidueye thanks to Pokken and Decidueye offers a niche as a dedicated archer that hasn't been filled outside of the Links and Pits having it for a single attack. Decidueye is also very popular both in Pokemon and Smash communities, but popularity really wouldn't factor in because I'd practically guarantee the roster was decided before Sun/Moon even came out.
You realize Mimikyu is also in Pokken too, right? Not that being playable in Pokken should be an indication of GameFreak's invested interest.
If you wanna talk about GameFreak's invested interest, why were they so obsessed with promoting Mimikyu? A song, a major role in the anime, a totem trial.
How is Decidueye's appearance in Pokken any more an indication of GameFreak's "invested interest" than Zeraora, who is going to be in the new movie? Or Mimikyu who is a member of Team Rocket in the anime?
When did Pokken Tournament become the end all be all? Like, if anything, shouldn't the fact that Decidueye is already playable in a fighting game kind of cheapen his appearance in Super Smash Bros?