Why the LP system?

You're grinding, trying to get to your dream elo, you are at some 78 LP and you win that game, But then you see it. You only needed 22 LP to make it into your promos, but from the last game you got 21 LP now you are sitting at 99 LP. You could have gotten into your promos by now, but ONE LP is the reason you have to continue and play another 30 minute long game. All because you were off 1 LP due to something you can't control.

I believe Riot should take away the LP system and implement some sort of bar, almost like your promo games. This would be a bar with 5 sections and when you win a game 1/5 of the bar gets filled in but if you lose 1/5 of the bar gets depleted. (and possibly have some grace games shown for when you are at 0 LP before you get demoted). And when you have the whole bar filled, you go into your promos. I feel like a more exact system would be much more appreciated and don't see why LP varies in random numbers based on MMR.