I do not agree with you, and I do not want you to get your wish. Here is why.
Doing things on my own is the more fun part of the game. Hugging my team 24/7 is no fun regardless of my role. It is no fun to need a dedicated bodyguard and it is no fun being that bodyguard.
Your idea of rengar being easy to counter depends on the support sitting on the ADC and saving his skills as an immediate response to rengar. In other words aggressive play-making activity from the support is punished by "the support is not guarding the ADC, rengar kills", and the rationale you give is "the support's job is to protect the ADC".
In other words, my fun as a support player is being punished by the Rengar player. The way to counter the rengar player is to suck the fun out of my game.
Do a short thought experiment with me. "Honestly, Rengar's job is to protect the ADC". How would you like Rengar to be that? a jungler who's definition is that he sits on his own team's ADC and defends him. "Rengar should not be wondering on his own, he should stay with his team". I mean, you don't think this is a fun way to play, that is why you pick Rengar, so why do you think I as a bot lane player should enjoy this game-play pattern?
So, here is what I think. As a bot lane player I should get to say what the fun in my position is, and then I should have reasonable access to having my kind of fun regardless of your pick (and the same goes for you as a jungler player). If you get to have a champion that heavily punishes play styles and champion picks that are bad or not focused at protecting ADCs, than even if you did not pick Rengar I have to respect that if I go for a non peeler you might pick a rengar.
Not peeling for the ADC and playing ADC with out the support holding your hand should be viable play styles, on the simple reasoning that they are fun play styles. Rengar punishes those way too hard, and he punishes them straight from champion select.
TL;DR: I don't want the support's job to be protecting the ADC, and I don't want the ADC to be forced into cohesion with specifically the support. It's much more fun to make mad plays with the jungler. "old Rengar" forces "protect the ADC" from the support, which is why he is a problem champion.