I literally never said that "fan favorite" picks are a "bad thing". Not once. I also never said that picks should be "purely pragmatic" and dependent on "sales and relevancy". Not once. I said that, "you've got main characters from some of the largest, and still relevant, gaming series." This is entirely different from saying "Smash additions should be purely pragmatic and dependant ONLY (never said that) on sales and relevancy". That's mischaracterizing what I said.
Waluigi could absolutely get in. He has fan demand. However, the context for which I don't see it happening isn't because he doesn't matter or something. It's because he is literally already in the game. He isn't a playable character, but Sakurai has shown throughout Smash's history that he views the position a character is given as...relatively solid, within the same game. Sure, Mewtwo and Lucas had their original trophies in Wii U, but that simply isn't the same thing as an Assist Trophy. We've never had an AT upgrade in-game.
The other reason I base my thoughts on 1st parties vs 3rd parties as DLC is that with Sakurai looking to, "expand on the record as much as he can" with the DLC, adding in characters that are not only already in Smash, but from a series that already has a playable character on the roster (or a boss, for example), does not expand the crossover record. Not to mention, if the 2nd batch of newcomers is a Season 2, structured similarly to Fighter Pass 1, then the additions will once again run into the situation of, "What spirits do they bring? What stage? What music?". Characters from series with immense amounts of content already in Smash will struggle with this. Series that don't have any content in Smash, will not struggle with this.
I simply don't believe that Sakurai will upgrade AT's or Spirits when he has the option to go all in on new characters. If he does, then that's his prerogative. I'm not the director, it's his game. But with this being an opportunity that will likely never be available again, I would think he'd go for a strategy that brings in the biggest names, especially when effectively most if not all major Nintendo characters are playable or in Smash in some form.