Ah yes the old "your below me in rank so you clearly dont know what you are talking about kid" thing.
Am i gold 3 yes.
Before i was gold 3 i was plat 1 99 for months and i couldn't break out of plat 1. Life happens, Meneires Disease happened in mine however and i am not able to play league as much as i used to and so my skills are far below what they used to be but my understanding of the game is still as sharp as ever. As a diamond player you should know that Nasus doesn't have to bee line straight for the ADC he simply has to drop Wither on them and that is enough to let someone else bee line for them or drastically reduce their DPS due to the attack speed slow and the fact that Wither much like Exhaust tends to bork up the ADC attack animation. Also as a diamond 5 player that you are you should also know Nasus isn't usually the one to initiate but he is the one who will most likely be causing close to the most damage outside of his ADC.
Btw since you are a diamond 5 player then you should know that there are only a few carries with attack range over 600 and 3 of them have that range tied to abilities only while most of the rest sit at or below 550 attack range. You most likely also realize that fights never go perfectly as expected in every single perfectly imagined scenario that you played in your head. So that carry if they ever want to hit Nasus eventually they WILL get in that 600 range Wither which will then proceed to either let Nasus catch up to them OR force them to blow cooldowns defensively.
The concept of how Wither works is also easy to see on someone like Nunu.
Simply run up drop E on them and do anything else that needs doing the Carry will most likely back off or try and commit to fighting you in which case both decisions are wrong and add a frozen heart into the mix and a Randuins Omen and i am pretty positive that that Carry is not going to escape without blowing cooldowns like flash or their escape defensively and from observing people play and helping coach some of my friends most people tend to play far too cautious once they know they have no defensive abilities to fall back on.