Sakurai clearly doesn't take [Zelda] seriously
We need to stop saying things like this. Zelda has substantial representation in pretty much every facet of the game - one of the only new stages in base game Ultimate, a ton of items, arrangements... Immense care was put into redesigning the Zelda cast for Ultimate and giving them each a distinctive look celebrating a different period of Zelda's history.
By now I wish that they would suck it up and add a supporting character, I think it's overdue and they could have justified adding additional villains a long time ago, but it's not a matter of not taking the series seriously. It's fundamentally the way characters are selected that discourages Zelda from building on top of itself, a hesitation to add characters who will not stick around unless they are concretely the protagonist. Since the major players are always Link and Zelda, and often some form of Ganondorf, Zelda does not have the same revolving door plan as Pokemon or Fire Emblem. Even though personally, I wouldn't mind if it did. I just recognize that this has a very obvious explanation that doesn't substantiate Sakurai secretly hating the series.
I recall everyone used to say Sakurai didn't take Metroid seriously, that he even spited it and hated Ridley. After Ultimate it's hard to buy into this argument anymore. Yes it was after substantial fan pushback, but Sakurai has gone on record several times over to explain that he wanted to add Ridley but couldn't make it work. In the meantime we saw a new character in ZSS, Ridley imagined as a boss twice over to acknowledge his inarguable significance. Zelda is at a similar crossroads, one where the characters can consistently be updated to fit the trajectory of the series but no additional supporting character has felt "right" yet. Maybe if we make our voices heard we can change their mind? But it's not productive to pretend it's an inherent bias, when it's not.
The only series I'd genuinely accept someone saying Sakurai doesn't care about is Sonic, lol. Even in my own frustration about the way one my favorite series, Punch-Out, is treated I recognize the effort that has gone into Little Mac's expansive alternate costumes, Doc Louis appearing in victory poses and so forth.