I know I seem negative a lot, but hear me out.
"Viability" is kind of a crappy term because it evokes a lot of different things to different people. And to a lot of people, being "non-viable" is essentially a death sentence, and no one wants to believe their character has no chance at all.
I don't think Mewtwo is even close to being viable right now. I also don't think it's impossible, or even improbable in the long run, that a Mewtwo will place highly at a national or major. After all, Jonny Westside just made top 8 of a notable SoCal tournament with the inarguably non-viable Samus. What I mean when I say "non-viable" is that this character has to outplay their opponent by a considerably greater amount to win than the reverse, in most cases.
The DK matchup is a really good example of this. A lot of times in this MU you can be winning every exchange and dominating the neutral, only to have DK live to 140, grab you at 40 (!!!) and Ding Dong you for the kill. It's not like we're Sheik and can just build percent like it's nothing--our frame data is about the same as DK's, we have virtually no disjoint to protect us from trading, and we get outranged. It is possibly the worst MU in the game; it's at least worse than Rosalina v Ness (and 1.13 Bowser might be even harder!)
So just get a secondary for DK, right? Seems easy enough--until you realize that your secondary does better against Fox, too. And ZSS. And Mario. And Yoshi. And MK. And soon you're using your secondary more than your main. Eventually you might realize that even Mewtwo's best MUs can be handled much easier with other characters.
There's another good definition of "non-viable" here, and it's tough to argue. If your sole concern is winning, there is no reason to pick this character.
Character loyalists will always exist. I mained Mewtwo in Melee and watched videos of Taj and I thought then like you do now watching Mew^2. "This character can't be that bad, he's just underrated."
He was that bad.
And you know what? At release, Sm4sh Mewtwo might've been even worse. M2K thought so, as did I. We're fortunate that we have patches now, but with patches comes increased expectations that obvious problems get fixed and that the gaps between the viable and the non-viable get shorter. Because although being non-viable isn't a death sentence and never really was, it still sucks.
We've got (at least, but also most likely) one more patch. Let's make the most of it and raise awareness about what Mewtwo still needs. If not to become viable, then at least to make it a little easier for us character loyalists to beat the odds.
Well, you certainly made your case well.
Trust me, I'm not against buffing Mewtwo! Don't get me wrong! Make Mewtwo S tier, I say. I'm all for it!
It's just... I dunno. I'm still not as pessimistic. Like, you're probably right... Right now, I'd say Mewtwo goes 50:50 with most of the cast. DK? That one might be bad. Bowser? I was leaning towards slightly bad, but I think it's manageable. Even Shiek isn't a complete shut-out. ZSS might be the worst Mewtwo matchup, perhaps... I bet that's pretty horrendous. I've never seen that matchup play out in tournament.
So, even if I'm right (and I might not be), a character that goes close to 50:50 with the whole cast isn't that great. Obviously, Mario, Rosalina, ZSS, Ryu and Shiek have better matchups, but... y'know, to me, Smash was never about winning no matter what. ZSS and Shiek have playstyles that I just don't enjoy or do well with. Mario's okay, but I'd never seriously main Mario. I'd main Rosalina or Ryu, actually, but both have a much steeper learning curve than Mewtwo. But why those two? Because I like those characters.
Taj was kinda my hero in Melee, too. Germ too! Germ was the only guy to ever make Melee Link look impressive.
There's a reason I stopped watching Melee last year: I just don't give a crap about most of the top characters, and the dated graphics don't help. Melee's old and tired. I don't play it anymore, and I just don't watch it anymore, either. I used to root for M2K and Hungrybox, and I still do, kinda, but I don't watch them play much.
If Smash 4 ever got to the Melee metagame's level of stabilization and centralization, where about 7 characters (give or take) rule the game, and all you saw were those 7 outside of pools... I couldn't put up with that for long unless I had someone to root for. I like Rosa and Ryu, but I'm not sure I like them that much.
I play Mewtwo, Ike, and DK because I love those characters. Pretty much all my favorite Smash 4 players main those, too. To me, having fun and doing well with characters I love is the best part of Smash. When the dust settles and the final patch is downloaded, that isn't going to change.
I'm too poor and my car's too dilapidated to attend tournaments, so you can imagine my outlook on the game isn't based on winning at any cost so much as optimizing what I have to work with.
By all means, I am for all the buffs I can get for my favorite characters. Right now, Ike and DK are sitting pretty in the high B/low A tier range, and I'm content with them. I don't know where Mewtwo is. Part of me believes he's in the same high B/low A tier range, but even so, there are clearly still hitboxes on Mewtwo that have yet to be fixed and other issues that should be addressed.
All I'm saying is, if Sakurai and team ignored us and left Mewtwo in his current state, I wouldn't be furious. Disappointed, but not without optimism. At least they gave Mewtwo a fighting chance.
Bring on the buffs and fixes though, I say.