Corki is like a top ADC and his siege is garbage aside from trinity force - and lich bane siege is more or less the same thing as trinity force siege.
I mean, I think we both agree that adcs are viewed as "required" when they shouldn't quite be, and both agree that siege and sustained damage are things that adcs are good at and things you need to make sure your comp has if you have no adc. I'm just saying that the statement "adcs are required due to siege" is absurd, because Ziggs, Nidalee, TF, etc. are so much better than Urgot, Vayne, Corki, etc. when it comes to knocking down turrets in contested situations.
I also think being able to kill a 6-item tank without being kited is a harder to replace benefit from adcs than siege, but I do agree with you that it's not ONLY adcs who can do this. I think it's probably just that there is a group of champs who do siege and sustained lategame damage effectively (call them "pure adcs" maybe, and tag Trist as the absolute best example). So when your comp doesn't need one or the other that much and wants something else from bot lane, I think these ad carry players are only sticking to champs that play like the pure adcs they have to be good at because it's what they're used to do. Or something. I don't really know why people don't break the meta sometimes, just like I don't know why people still take wota first item on Vlad. I guess the game is just so gosh darn big that people use herding at the high level to efficiently figure out what to learn and they miss some good stuff. Who knows.
All I'm saying is that if you had like, a Jayce top and Ziggs mid, and then your bot lane didn't do adc for whatever reason, it's bonkers to think you'd be sitting around sad because you're not good at siege.
