Sivir and the Balance of ADCs
Sivir's getting picked or banned almost every professional game, and is seeing quite a lot of soloQ (really strange thing a while ago). Her ability to engage or disengage teamfights, good amount of AoE damage and short-range kiting make her a beast for both teamfight and pick-based comps, and you can also focus your resources in other lanes, staying with the utility. So, a lot of people start talking about future Sivir nerfs. But there's the problem
There should be no nerfs.
I'd be lying if I said Sivir ain't a top tier ADC right now. She's really good at her role of "Utility focused carry", with her team boost, medium survivability (spellshield+movespeed) and AoE damage (for both waveclear or teamfights). But she's also pretty bad at 1v1 duels (hence the Vayne counterpick), has no straight escapes and is heavily countered by high-armor builds. Compared to top tier picks in other lanes (Azir, Cassio, Rek'Sai, Gragas, Rumble), she's healthy. The big problem is that most ADC champions are fairly weak, either because of low numbers or because they lack a clear identity inside the game.
Lack of Identity
To exemplify this point, let's pick some low-tier ADC right now. Twitch.
Twitch is a champion we've always seen in pick-based comps. He has a stealth (safe way of engage, surprise factor) and a stack-based explosion (burst!). But, as a big difference with most pick-based champions, he has no gap closer nor hard CC. Yes, being invisible gives you an advantage, but unless you engage against a low mobility champion like Jinx/Kog'Maw it's really hard to kill it before it goes away. His base numbers are fairly low, so he depends a lot on getting an early advantage or he won't be able to pull out this picks. His ultimate is a teamfight oriented weapon, but it requires all your team to fight around you, spending their CC peeling for a guy who's fallen behind and will take quite some time to kill the enemy team. With this kit, Twitch is based in picking isolated enemies, but doesn't have the tools to play from behind nor to pick up an advantage just on his own, and his teamfight is really poor unless the team snowballed out of control (hey, wonder why SSW used it so much...).
I could make another example with Ezreal. While he's considered a mid-tier pick right now because of the strength of certain poke champions (AP Kog/Varus), he has quite some struggle by himself. He has quite an established identity in long-range kiting with his spells: his Q is basically a skillshoted 1150 range AA, and his E allows him to relocate VERY safely. His waveclear is really low and depends mostly of his ult, which is also his only source of AoE damage for teamfights. But what about his W? Well, if you're AP Ezreal then it brings tons of AoE damage to champions (80% AP scaling!), but it won't deal any damage when you're an ADC, specially because you'll probably max it last (shorter cooldown on E is very useful in midgame). On the other hand it brings quite some team utility (40% attack speed buff, and can be auto-casted with E), but if the idea of the champion is to be played in a poke comp and kite around the enemy team, that's not a useful buff. Would expect it more in a 1v1 splitpushing champ as selfcast, or in an AoE based teamfighter. If the idea is to not give him extra waveclear (because otherwise he'd just be another kind of Corki), then give him some damage (to prevent him of being too ult-dependant), or give him some way of sustain so he can out-trade other poking champions. Right now he only fits a comp based around poke, and finds it really hard to come back from behind when losing lane.
Won't make it too long, but you've got the idea. I really think Sivir is in a healthy spot, but few ADCs are. Jinx and Kalista are the other two to instantly come to mind (maybe Kalista having a little too much AD scaling), and Corki/Ashe sit pretty well in their role. Other ADCs had to be number-nerfed in order to tune down their overall strong kit (Lucian, Graves, Draven), moved to midlane so they could be more safe and effective (Kog'Maw, Varus, Urgot) or face the same identity problems we've talked before (Miss Fortune, Quinn, Tristana to some extent). Don't kill the healthy champions, tune up the role, because ADCs are really underwhelming these days. Thanks for the read, and hope this gets to be useful.