Quick question, are Mipha and Urbosa the most popular Champion because I recall Aonuma mentioning them in an interview around the start of this year? Those would be the most likely Champions I would assume.
As for your argument. Possibly attract a few buyers, sure, but we could say that towards any unique SSB4 newcomers and prospective Smash Switch newcomers not named Inklings which got confirmed early on for a reason. But few doesn't mean a lot and there's not going to be a backlash for not including a Champion if there wasn't for K. Rool or Ridley's exclusion last game. At most, some people will be disappointed but they will buy Smash like everyone else will. To get a backlash, you have to cut the Marios, Pikachus, Links, and Kirbys and even in a rebooted roster, we're never going to lose that in a healthy Smash future. That said, I will entertain the tie in idea.
If they want to take advantage of tieing in BOTW to Smash, all they need are Link, Zelda, and Ganondorf with an emphasison BOTW Link and possible moveset changes alongide ATs, a new stage, and items. Aside from the possibility that a Champion becomes an unlockable we don't hear about until after release (theoretically), if they do get announced beforehand, the best way to ensure that Mipha/Urbosa gets noticed is to make them the focus of any BOTW tie-ins to Smash and why BOTW fans who aren't fans of Smash may want to play as them in this game. Otherwise they'll just be there as another newcomer, not anymore special than your typical Nintendo character. My question is, would Mipha/Urbosa's role in BOTW and popularity be significant enough to be a selling point like Zelda and Sheik were in Melee?
Also if another Zelda characters is not needed, then they are completely "unnecessary". It doesn't matter how good of an addition Mipha/Urbosa would be, they are not so crucial that Sakurai can't afford to skip out on them. This is not something that applies exclusively to them, not a single prospective newcomer has to be included because everyone has a reason to be skipped over. In fact, I would argue that half of the current roster are easy cuts because sorry to say, the likes of Lucas, Roy, Bowser Jr. and Mr. Game & Watch aren't going to deter a lot of people from the next game if they don't show up and I say this as someone who want Lucas to be a mainstay. The reality is that a reboot is coming and while it may not be this game, it could be the next and we'll have to prepare for many painful cuts when that time comes. At that point I don't think discussing if the next one-shot Zelda character will get in Smash will become the most important topic.