There is an automatic system to detect leavers, it even puts their name in red in post game.

Why should I still lose 21 LP when I played the whole game down a man?

I don't cry when trolls lose me LP because detecting trolls is difficult in a quick LP gain/loss at end of game. But leavers is pretty much a guaranteed loss in Bronze tiers solo queue (I think Ive won once or twice).

I'm not saying I shouldn't lose anything, it is still a loss, but the system should take it into account and even just cut it by 20% or one fifth of the team.

Thanks.