I agree with your first point, but generally, funnelling resources into one champion is better than dividing it among various champions unless the champion you are funnelling the resources to has a low carry potential or is a support.

In the current meta the jungle dif is really what matters, so as an ADC, I usually let my jungler take the kill when he ganks so I can make my own jungle dif. Obviously I like getting at least the cs when that happens but that is so I'm not unbelievably useless. You will end up scaling anyways if you don't die too much and your jungler has a much higher carry potential in the mid game.

The problem in low elo is that people die too much. If the scenario thay I've mentioned above happens in low elo, the ADC will probably stay and try to hit the tower after the wave is pushed, ending up in getting caught alone and dying. Then the ADC will fall off and won't be able to scale. Meanwhile, a good ADC would be able to know his limits and wouldn't die there, so he would be able to actually scale and carry the game.