It is very obvious when you have an autofill support. They don't build a sight stone half the time, they usually pick an AP carry, and they usually flame other lanes.
When the other team has a support main playing a real support and building support items vs your troll support the chances of winning are very slim.
I'd love to see a spreadsheet of Win Loss rates between an autofilled support and a preferred support. It's such a massive imbalance in ranked with this system.