Technically everyone is viable, at least from what I've played, you just need to learn how to play them properly. Yes, some are better than others, that's a given, but ultimately, if you put enough time into any character, you can hold your own against the best. But of course, I'm not in the competitive scene so obviously it's different when it comes to balance and viability. This is just what I've experienced with online matches and playing against friends and family. At any rate, I doubt we'll see smaller gaps between characters in future Smash games given that the characters aren't suppose to be balanced in the first place and haven't been since the first Smash game. It's not like that will magically change in the next Smash game just because the competitive players ask for it. Sakurai and even Nintendo don't see Smash Bros. as a competitive game as many others do, even if Nintendo has been openly supporting the community, it does not mean the balance of characters will suddenly change. The majority of Smash Bros. players aren't competitive but casuals, that is a fact. While the community is very vocal, it is still a minority at this point in time and even then, Nintendo has never been one for balance when it comes to games like Smash and Mario Kart. It's more about the over the top gameplay and characters rather than fine tuning the balance of the gameplay to them and I don't see that changing anytime soon.
Yes, I know they are not about balancing. But let's say something: say some random character was busted, even better than MK, would you think Sakurai would put a balance patch? I would want a patch, depending on how bad said character really is. He said he'd fix large issues, and the community is turning Diddy into one(even thoughhe's fine, I see YT videos about diddy being Icies tier).
Casuals do care about viability. I'm likely not competitive, but I don't want to have my favorite character being not good. The Little Mac, Robin and Bowser outcries were definitely not from the tourney players, and the Diddy outcry started with casual players, then ZeRo got to him, then...
It's agreed upon that there are characters that need buffs, some that need nerfs. And in any game I could train with Ganon for 10K hours, maybe i'd beat ZeRo. Maybe.
I'd probably still get bodied by other players because Ganon is huge, slow, not getting nearly enough reward for landing hard reads(AKA his whole kit), and lacks methods of killing despite being a heavy, as well as getting gimped easily, and leaving opponents alive to longer % then lightweights do, because his whole moveset consists of hard reads bar one or two moves...
Nintendo has said multiple times that they are willing to cater to the hardcore(in a more cartoonish way, hence why Fire Emblem still exists, and why CODENAME STEAM and Splatoon are coming/have come). Sakurai is the one here who wants to cater to casuals. That's fine. Absolutely fine. What I disagree with, is the way he says that he won't balance a game because we want chrarcters to all be the same boring stuff. It's easy to understand how he thinks this but it's wrong. Most of us here don't think that.
Casual players do care about balance, or else Diddy would be C tier, Rosa and Mac wouldn't have been nerfed and so. Casual players can literally decide what the bandwagon top players use by getting vocal over what they hate. They have far more influence than us by virtue of being and acting casual, because Sakurai.
Like I said, I don't want a stale bagel. I want to keep this game the awesome, colorful shrimp platter it is, just making every single part taste good. I don't want a stale balanced game, I want a variable balanced game. Oh and by balanced I mean everyone clearly fun and viable. It's impossible to make it happen, but hey, you can hope right?