I can understand not being optimistic about getting non-Triforce characters. Though Sakurai has never confirmed the suspicion that he'd be against non-Triforce characters, so it's only speculation that he'd be against them in principle. And even if he was unilaterally against them, we are indeed possibly on the precipice of Sakurai taking some reduction in his role, so that issue could end up resolving itself at some point.Realistically there shouldn't be any reason at all, but bizarrely it hasn't happened yet and I think I can suspect why. I love Midna and Skull Kid and would love to see them in Smash, but I can't help but think that the triforce wielders being the only playable fighters is by design from Sakurai and not due to other circumstances. It's been over 20 years since Melee and Smash has skipped over the addition of popular Zelda characters that came with every release even when the game releases were perfectly timed, we've had three Smash releases since then and the only newcomer was Toon Link (another triforce wielder) back in Brawl. If a new director takes control for the next game I'll be hopeful for more Zelda newcomers, but if it's Sakurai again the best I'll expect is a revamped Ganondorf.
I hope that they are at least aware of the demand for different Zelda reps. We know some like Skull Kid and Midna had some numbers on the ballot, and one can hope that they looked at combined votes for the category of Zelda reps much like they almost certainly looked at combined votes for, say, Castlevania characters, in the event that no singular Zelda rep stood out enough. And as I was saying there was also the explosion in Skull Kid support the minute he had some theories/rumors in his favor during the leadup to Ult, which could have been noticed even if those theories/rumors were very unfounded from the start.
Regardless of the likelihood, I think reconstructing the Zelda roster almost entirely would be best. Like, when I look at it, I'd literally be good with it if they just kept Link and Zelda and started from zero with everything else. All due respect to him of course, but Sakurai's way of handling this series has just...not worked IMO.