If you win a game at 99 LP. you are placed in your promos with one win.

By that same logic (since remember--it's the same system that does shielding) "if you lose a game at 0 lp you'll be immediately demoted" I don't think many people would be happy about that.

True

Once you are in the promo you'll get one guy on your team afking in one game and in another you've got someone who totally sucks or is intentionally feeding to keep you from getting promoted. You are also going to get someone raging hard too. So I like to think of it as a way of knowing I'm about to get elo-screwed by riot.

Pro tip: never mention your promotion status to your teammates or you will get trolled.

Then why not 98LP? Then why not 97LP? There will always be the person who's just 1 LP off of the right spot.

Better to just make it a continuous system.