I think people really overemphasize Sakurai's preference towards his own run on Kirby. The fact its more generally represented in Smash largely has to do with elements that debuted in those games and became series staples (such as Kirby, Meta Knight, Dedede, and most of the Kirby spirits), content that comes from Smash 64 and Melee (such as half of the stages and music from Kirby in Smash Ultimate), and content getting reworked at the last minute (such as the Mario Land stage becoming Dream Land GB to give Kirby a stage on the 3DS version and Epic Yarn getting salvaged into the Great Cave Offensive after it was decided Woolly World would get a stage).
Now that I think about it, I don't think Sakurai really had a chance to genuienly prove whether or not his apparent bias was waning or not.
Case in point:
- Only the Mario and LoZ series got a new stage in Ultimate's base game
- All of the bosses aside from Galeem and Dharkon aren't based on particularly new faces
- Of all of the new items, there are only 6 which aren't Smash-originated; furthermore, they aren't based on any relatively new concepts or ideas
- Any Nintendo-Focused Assist Trophies are based off of long-existing characters (aside from Splatoon and ARMS, of course)
Representation was more expressed through music (but even then, there were only a minimal of remixes for certain franchies) and Spirits (although, they're basically lesser Trophies).
Given that "Everyone was Here" was the main focus of Ultimate, naturally it make up the majority amount of development time in addition to the inclusion of brand-new fighters altogether. Everything else in terms of representation would theoretically be less of a priority.
Therefore, I wouldn't be surprised if Sakurai wanted to fall back on what he knew best about the Kirby franchise in order to make development time run smoother. Same thing could possibly apply to other franchises in terms of the ratio between representation for the most recent entries vs previous entries in the franchise.
About BWD, some people already said what I think: there are so many other characters Nintendo could chose, I certainly don't think they would pick a 4th character for an already represented serie. And that's even without taking Sakurai bias into account.
Yeah, there were so many characters Nintendo could've chosen yet they went for an 8th Fire Emblem character which was clearly already represented in Smash.
On the other hand, they're not just going to choose any character since FE:TH was at least a very recent game at the time the DLC was established (and they wanted something fresh to promote), so I say that Bandana Dee's best shot would be to promote a new Kirby game in the (near) future. Heck, the next core Kirby game was said to be the "pinnacle of the series" so that would probably be worth taking advantage of in order to elevate the Kirby series further.
But the problem definitely doesn't revolve around whether or not a series has representation or not, anymore.