TL;DR: I agree about Malzahar (at the end), but I think you're overreacting or uninformed about all of the champs you listed.
In order: Ahri, Cait, Janna, and Jhin are generalists with many strengths that are looked for in their respective classes. They are also have a low skill floor, so even people new to those champs will perform decently with them. So yes, they will always be good picks, but not great. When they have been great, they've been nerfed (you can check their histories on the wiki).
Vi is similar, but she is the one who was most recently too strong, and perhaps still a little too strong. Her current strength is dependent on things that are/were in flux though, namely Courage of the Collosus, and the Season 7 jungle changes. Riot has been making changes to those, and they didn't want to double nerf Vi before seeing how the other changes affected her.
Regarding "piddly taps", you seem to have a double standard. Do you want those champs be nerfed harder until you don't see them anymore? Because that is the state of the last set of champions. Besides, I'd say all of those middle 8 champs are balanced (assuming the new Jayce nerfs and Kha'zix nerfs hit as hard as I think). Hecarim, Mao'Kai, and Poppy were only strong when CoC was broken. The others are only strong when someone is really good with them, and/or their opponent doesn't know how to deal with them.
In contrast, most of the champs you deem "in the gutter" are there because there is no good way to deal with them.
(I'd actually say Kennen belongs down with the other "gutter" champs, btw. He was only ever strong in Pro play because they could abuse his team-fighting strength, and when played right, the only way you could shut him down was with exhaust.)
Aatrox is not a fun or fair champion. If he's suffers greatly from S2 fighter syndrome, in that he has no weaknesses and can 1v3 constantly if he's strong. His healing and lack of mana means that he never has to stop fighting, so if he's strong enough to win fights consistently, he snowballs way too hard. This is only exacerbated by the fact that he gets more healing from damage items (AD and AS make his W better).
Azir, as well as Kalista and Taliyah, have way too strengths. AND those strengths are gated behind very steep mastery curves. Which means they are either garbage in normal play, or 100% banned in pro play.
I don't know what the deal with Cho'gath is. I guess he has too little counterplay to be allowed strength. None of his abilities can really be stopped/avoided except his Q.
Fiddlesticks is an unfortunate soul who doesn't have a good place right now, since the new jungle has not catered to him at all. They did buff him already, and I think they want to help him some more, but don't know how yet.
Kindred runs away with games if she's strong, because she is basically unavoidable as a jungler. Her mobility, undodgeable attacks, and safety due to range/kiting means that again she's has no counterplay. Are you seeing a pattern yet?
Morde has a weird identity but actually isn't weak. He's niche and not many people play him, but those who figure him out tend to wreck with him. Also, he's a juggernaut with no range, CC, or mobility, and relies entirely on his team to get him next to his opponents. So he pretty binary too - if you're next to him, you die, if you can kite him, he does nothing at all. Buffing his strength would make him even more binary, and buffing his weaknesses would bring him dangerously close to Aatrox territory.
Nunu is just entirely uninteractive. He spends most of his time counterjungling. If he wants to fight, he just clicks on you and permaslows you. If you want to fight him, good luck catching him or getting any damage to stick. Not a champ you want to see being meta.
Tahm Kench has half of Nunu's problems and half of Kalista's. For a while he was a no-counterplay top-laner. Now he's a support that no one knows how to play optimally except the pros, since his team-utility is only really appreciated on that level.
And Vlad is just a mage version of Aatrox. Constant healing and damage just can't be strong without being oppressive, especially when it is point-and-click and ranged. And he can be untargetable. Again, the only counterplay is to out-range him somehow.
But where does Malzahar fit in relation to all this? I'm not sure. I know he doesn't quite belong with the gutterfolk. But he's not all that fair either. Well, until you get QSS that is. I do think they missed the mark with the W cast-delay, but there's still time for follow-up changes. We still haven't seen how bad the changes impact him though either. Maybe it isn't as bad as it looks on paper. Idk.