I'd rather have them buff the other carries rather than nefing the good ones. IMO, every character should FEEL overpowered, and I have a feeling that more often than not, Riot sees problem champions and says "better nerf them" rather than "how can we make other champions play better against them?"
I mean, Sivir, Ashe, Kog and Urgot were all nerfed within the past handful of months, which led to the relatively untouched Graves/Ez/Corki becoming more dominant. Honestly, did they change anything about Ezreal before he started becoming popular?
Riot's penchant for the nerf hammer leads to these situations where some champions become extremely viable and popular out of nowhere because Riot nerfed all the other popular alternatives (ex. Rumble, Renekton).
Look, it's nice to want to buff everyone but it just isn't reasonable. There is a baseline for where champions should be that many of the AD carries are just above. It's partially why the role is so important right now. It's also an unrealistic amount of work to balance every single champion, and it potentially requires items to be retuned as well. Rumble and Renekton are both in a good place right now following their nerfs, so I don't know why you're bringing them up. Renekton has seen buffs and nerfs in various places, so he doesn't even fit as an example.
Corki, Ezreal, and Graves became as popular as they did because of a meta shift anyways. The mobility they had and the ability to jump walls simply offers them a huge leg up on the competition in many situations.
To be fair, Yorick as a support "makes sense" in the idea of his late game play revolving around his ult.
Yorick doesn't revolve around his late game, and one ultimate isn't enough to justify the presence of someone as a support. There is a necessity for supports to bring CC, and Yorick simply doesn't offer that. His strength is as an unmovable and undying tank who provides some amount of utility to the entire team. He's a rotten support though because he needs to scale to actually be useful. Why would you pick Yorick as a support instead of Lulu? She offers more slow, a longer distance harass, more CC, and a huge boost to survivability.
Sivir's kit is good but I always thought it was her unsafe range that kept her down? I don't know if the holy trinity being taken down a peg will be enough for her compared to just more Ashe and Kog. At least as far as competitive play is concerned.
Her range is fine. She has better kiting potential than many other AD carries actually because of the speed available to her from her ultimate and her passive. There are AD carries with lower AA range like Urgot.
If you doubt Sivir's viability in competitive play know that she was actually banned in a Korean tournament today.
Graves had been nerfed 2-4 times previously, but I'm not aware of any buffs to Ez or Corki that made them suddenly become dominating aside from Pulsefire Ezreal (which might have added some slight timing changes that made things better, but presumably people like because he's cool).
There was a mana cost reduction to Ezreal's W, which made it more viable to level that up first. The mana cost on his ult got bumped down too.
