On one hand, I can see why people think SWSH might not get a DLC character because of the spirit event. On the other hand, none of the spirits were going to be playable fighters (let's be real about this), and we know SWSH has an extended life thanks to the two DLC packs. Perhaps Sakurai knew that a new Pokemon project is happening in 2020/2021 but didn't know it's details and decided to go with the spirit event for SWSH's release preemptively, who knows? It wouldn't be the first time that Sakurai reserved a special spot for a Pokemon character years in advance, a Pokemon slot was planned as far back as 2015 for Ultimate, Sun and Moon didn't even get announced back then.
Maybe the fighter could be from SWSH's DLC, or a Pokemon game coming out in 2021, either way disregarding Pokemon completely because of the spirit event seems like a mistake that confuses Pokemon with other franchises. If there is one franchise besides Fire Emblem that gets special treatment from Sakurai and Nintendo it's Pokemon, it ALWAYS gets a new fighter in Smash, Sakurai admitted this himself when he talked about Incineroar and the process of how a Pokemon slot is reserved years in advance. I wouldn't even be surprised if the reason the DLC is taking as long as December 2021 is because they're spacing it for the new Pokemon game to get close to release so they can release their promotional Shillmon as close to the game's release as possible.
This is coming from someone who dislikes the new gen so there is no bias from me, the only Pokemon from the new gen I kinda like and would welcome as a fighter is Toxtricity, other than that the rest of the gen 8 Pokemon don't interest me enough for Smash.