Rosalina was added because Sakurai wanted to add a puppet character, and she best fit that idea. Further, she was from a game that had insane levels of popularity and she had appeared in notable spinoffs before she was added to Smash.
The Champions aren't a valid comparison. The success of BotW, while expected to a degree, could not have been predicted by Sakurai in advance.
As I said in my previous quote, there was absolutely nothing Rosalina had ever done in the games that would've caused Sakurai to automatically think she'd be a puppet character with Luma. The concept is so unique that there's no way he could've come up with something like that on the spot; her moveset was something he had to sit down and really think about, meaning he didn't write her off just for being too new. Yeah, her game of origin was popular and it had been out for a while, but just like the Champions,
she was still pale in comparison to other more iconic options. Daisy, Waluigi, Toad, and Bowser Jr. had been making dozens of appearances in Mario spin-offs for over a decade. Rosalina had only appeared in four games (SMG, SMG2, MKWii, and MK7). For a Mario character, that ain't much.
It doesn't line up perfectly, but I think the comparison works. You have two (technically five) characters from a major Nintendo franchise who a lot of people wrote off because there was no way in heck they'd get in before the "more obvious" choices. The biggest difference between the two is that Galaxy had been out for a few years, while BotW was unreleased. You're right about Sakurai not knowing if BotW would be successful, but based off the timing and the reception to the game at E3 2016, something tells me he'd at least want to be aware of any potential newcomers from this game.
Further, each character has the story focus vaguely on them for less than one fourth of the total story. That's it.
The champions have nothing except relevency going for them, which is something no character has been exclusively added for save Pokemon, which is a special case that has been explained.
And aside from Link and Zelda, who else from BotW got that much screen time? Even if they weren't center spotlight, they're still a major part of the story in BotW. Honestly, it's no different than someone trying to make a case for Impa by using her appearances in OoT and SS.
Just relevancy? Relevance don't mean squat if your game wasn't notable (Just ask Henry Fleming and Tethu). Characters also have to have moveset potential for Sakurai to consider them, and that's something the Champions certainly have. However, even if we focus on the Champions' relevancy by itself, that's still one of the most important factors for Sakurai in choosing newcomers.
Almost the entire list of Smash 4 newcomers were relevant around the time they were chosen, many of which actually debuted in the time between Brawl and Smash 4. Even if relevancy really was all the Champions had, that's still a pretty big advantage, especially since a majority of their competition is relying entirely on a spin-off game.
Roy? Easy clone material. Corrin? Promotion for an upcoming game. Alph you can almost argue, but even then, he's only a costume.
Champions? Promotion for fairly new and popular Zelda game on the Switch.
Seriously, you can make a case for practically any character with this method. It's just randomly picking a reason why a character would make sense in Smash.
Skull Kid? One of the most iconic Zelda villains of all time.
Tingle? Only other Zelda character to get his own game.
Impa? One of the only recurring Zelda characters not on the roster.
Explaining why a character is on the roster is one thing, but making a case for that character long before they even get revealed is completely different. I can guarantee you plenty of people would make a counter argument for Corrin if you stated that back in mid 2015. Likewise, any of these Zelda characters could get in Smash, and everyone would act like it was the most predictable and understandable decision Sakurai ever made.
If you want to argue that one of them is extremely unique and would draw Sakurai's attention, then...who? What about any of the champions is so interesting that it dwarfs the likes of Impa, the most recurring Zelda character not already in Smash?
What is it about Impa that's so interesting that it dwarfs the likes of Skull Kid or Midna?
This is all about perspective. More specifically, Sakurai's perspective. I couldn't tell you whether or not Sakurai would think the Champions are more unique than Impa, but I can tell you that this would be Impa's fourth attempt at getting on the roster.
If Sakurai couldn't add Impa back in Smash 4 when she was actually at a peak in her relevancy, why add her now? She ain't getting any younger, and with all the newer competition she's picked up along the way, getting in Smash is only gonna get harder and harder.
I've supported Impa and just about every other Zelda character at least once, but so far, it's been to no avail. That tells me that Sakurai doesn't see what we see in these characters, because if he did, they would've gotten on the roster a long time ago. Instead of banking on the same characters, I'm taking a step back and looking at things through a different perspective, and that perspective tells me to start looking at the atypical choices.
I'm having a hard time taking them even remotely seriously when you compare them to their competition...which is most of the Hyrule Warriors cast.
Likewise, I'm having a hard time taking Impa and the others seriously when they're relying on a spin-off game, one where virtually every notable Zelda character has to share the spotlight with others, to be enough to trump BotW, a highly anticipated game that was on everyone's minds when Sakurai started development.