Basically, out of every single Smash Bros franchise, F-Zero is the least loved. Having only one character through the near 20 years its been up. Almost every n64 character got their own newcomer... except for F-Zero. This is also after GX, so Deathborn or Black Shadow would be expected. However, due to poor sales of those games, they never bothered (but then put Lucas in for some reason).
Cut back to now, where Falcon is the only F-Zero racer. There is only one "popular" F-Zero character that could place high on the popularity polls, Black Shadow. However, there's a pattern with Ridley and K.Rool that I think will play out. There's a villian newcomer for each N64 character. Then they get an additional echo. This should mean that Falcon will get Deathborn as a newcomer and Black Shadow as an Echo. I also don't think that Deathborn could ever be a boss as well, since 1. He's small, 2. He's a playable character in F-Zero GX, 3. He's kinda set up to be the main antagonist of F-Zero GX. Also Deathborn was not only considered one of the few F-Zero characters that were "Trophy worthy" but he was Falcon's All Star Mode Hard unlock. Said Smash 4 trophy also states that his status as a threat is still over Black Shadow. While the Smash 4 trophies for F-Zero do contain a weird combined anime/game lore, there is an odd thing for them to suddenly change it like that (unless it really is a localization mistake).
So, I don't think that this is just Sakurai dragging a dead horse out into a race for once, he's actually going to try to feed it hopefully...