You are simply not right dude. First of all, when it comes to Diamond 1 and Master games where someone can really be so God damn god on a specific champion isn't so hard to check if there is some Zed main queuing at the sane time you did. You can easily predict it and ban his Zed, Yasuo or whatever. I often watch sone good streamers on Twich and yes, they do that, I remember when I was watching Azoh playing Zed, he ALWAYS banned that Kha'Zix because one of the besyt master Kha'Zix was online and guy even asked them why the hell are they baning Kha'Zix and Azoh answered him that he know he is playing. There is a small number of players in really high ELO so it is not hard to "track them down".
When it comes to the actuall gameplay, no, Assassins do offer a lot of counter play. As a Rengar and Kha'Zix main, I really hate when enemy team is grouped around their ADC, I am really not that mad to press R and just rush in and kill him...lol, everyone will see that "!" and get ready to unleash the Kraken on me as soon as they see me midleap. In the other hand, Kha'Zix is even more predictable because ypo can see him, Talon also, his ultimate is really well telegraphed and once you see it everyone can get ready to protect you, any aoe stun on top of ADC as soon as Talon appears and BAM.
When it comes to "solo man ADC" who pushes bot alone at 30 minutes marh, well, he really deserves to get deleted by a Rengar, Kha, Talon, Zed or whayever because hes doing something he is not supposed to do and if theres Enemy rengar, why the hell should you even think about going anyehere solo?
I literally do not see a single problem there. I am not biased because I play and enjoy a lot of other champions so I dont have to protect Kha'Zix for ecample. you are simply wrong and that kind of spell or an item you are talking about is something Riot will never make. It doesen't affect only assassins but literally everyone and as I said, noone should ever get punished just randomly like that because of someones else misstake.