Botlane would become SO much more toxic. An ADC can't get S- without his support performing well too, so if the support messes up, the ADC will yell at him for making sure he can't get S-. This change, while making climbing a lot easier if you're good, will invite toxicity into more games because not getting S- means you're losing more LP than if you got it. It's labelled as positive reinforcement, but it'll be understood as negative reinforcement. "If don't get S-, it's because you're bad and therefore you deserve to lose more LP."
The current rank system works. If you play enough, you'll be placed into whatever division you deserve to be in over time. If you're better than the people around you, you'll win more than 50% of your games and can therefore climb. That's what the system is there for.