Ranking up based on how well you do

So why is it that every time a player loses, when they do well, they have to lose LP?

They could get an S ranking and still have to lose LP

Why not reward them for getting an S, because we know that that shows that they did well. If they do well in a game, if they aren't the reason for the loss, then why not give them LP. Conversely, if someone does really poorly on the winning team, why are they awarded LP? Why not take LP away from them or at least decrease how much they get?

Let's say in promos you have to win, but otherwise you have to simply do well.

Lets say that if you win, you gain a flat LP based off of your MMR and all that other fun stuff plus or minus a certain amount you get from your ranking,(S tier is + 10 LP, A tier is no bonus LP, B Tier is -10, C tier is -20, and D tier is -30, but of course could be capped so people aren't getting 45-50 LP for a single win)

So let's say I win a game, I get 20 LP for winning and I got an B- so I get 10 LP. Yeah you still get LP, but you get docked for having a mediocre game.

Say I lose a game and I get -15 LP, but I get an S- because I did really well, so I'd instead get 5 LP. Because I did well, even though I lost, I still get the benefit of progressing up the ladder.

This isn't very abusable, and would in fact be relatively beneficial for players. The opposing team has no incentive to allow you to get an S, so you really would have to earn that grade. If you do poorly, but still win, you aren't holding back the players who are better than you, but instead are forced to either get better or sink. It also promotes more teamwork, so that there's a greater competitive nature to this game.