Im a supporter of BWD, but 3 argument that the game explain team didn't present was the case of how implementation would have worked in his favor, hals recent push for him, and not including the smash ballot and fan service.
Im no game developer, and im sure as hell am no sakurai, but wouldnt it be easier to add in BWD, since they already have a working rigged model, and several bits of animations, of Waddle Dees. They have both high quality models thanks to Dedede entrance and win screen, smash run uses both parasol WD and waddle doos(and even if smash run dosent return they still have the animations of those enemies), and possible leftover bits of animations from dededes old side smash and final smash.
Imho, they would just need to gather all of those things together, polish the animations bits, or use kirby as a base to finish some animation, and slap a bandana on his head and thats like a good portion of his implementations finished by using old resources. While the counter argument could be made that sakurai may not like the idea of reusing old assets to being in characters, smash has had a history of utilizing old assets (be it clones, captain falcon reusing an okd character model in the 1st pitch of smash bros, wireframes, ect.)
Hal also has push the character in almost every game. While its ultimately up to sakurai to decide to put him in or not(or in the case of Corrin, have no choice but to do it) there push for the character over the years has not gone unnoticed, even if people want to act like he isnt anything special (see resetera reaction to the video).
They negeleted fan service and the smash ballot as well is a problem. While he may or may not have been the highest in the ballot, he was most likely the most popular kirby character during the ballot. And thanks to source gaming, we know that the ballot will be used to determine the roster for future entries. I felt as though this point could have brought up a good argument for the character(and future characters that relate to this) but that they didnt felt like a wasted oppritunity.
I know allot of effort has been put into these videos and its a wip but man, i am not feeling these at all. Its already one thing that i dislike people speculating characters and saying words like "deserving" to justify a character to be in or not, but its worse when people of considerable influences(ala youtubers) do it. It brings unneeded conflict within the community and Knowing the smash fan base they will(and already have) taken these video seriously. And to me... Its tiring to see these arguments time and again from the community thinking that they know what nintendo will do with this game, when we all know this is the most unpredictable smash game to date, and most likely the community's conceptions of characters are almost always destroyed(have we learn nothing from "too sexy for smash", "no guns in smash", "must have a major role in an nintendo game", "4 characters per series", and chrom.)
As a final note lets all remember that not 1 person or group of people have a final say in the smash brothers. At the end of the day gameexplain, while i dont agree with the video verdict, nor am i a fan of these types of videos, are just youtubers on the internet expressing there opinions. Sakurai will do what he wants and what he wants to do is please the fans(not to sound like he only lives to serve, but to satisfy as many people as possible), and the opinions of people like youtubers, reddit members, smashboards, twitter, gamefaqs users*, and resetera* are just a very small part of the nintendo community as a whole, no matter how bad they are (especially those with * have very VERY terrbile taste).