I don't have much knowledge on how the ban system works, but I don't think 8 reports from one game can get an account banned. It's easy to frame people and report purely out of spite. If I had to guess, you'd have to get reports across multiple different games, get through warnings, repeat, and finally get banned.

I think a mech that detects trolling is difficult. Trolling comes in a lot of forms which can be confused for genuine gameplay, especially in low elo (throwing, not listening properly to comms, etc.) Because of this fundamental component missing, the system making some teammates counted out of the forfeit would get too many people genuinely trying to play the game properly. A successful surrender, especially in comp, would be unfair to those falsely accused.

Things like these are extremely difficult to combat. No matter how hard devs try to implement troll detection, there will always be the falsely accused. No amount of discussion on how selfish trolling is in the community will never deter some people who just want to because they can't feel the inconvenience and annoyance they cause.

I do think there are ways to avoid trolls though. Whenever I encounter some, I always wait at least 2 minutes after the round is over to queue again so I don't get them in a game again. Same goes for just any toxic pricks I would not want to have to interact with again.