The attraction of the league system is that you get promos in exchange for a safety net.

But that's not true.

You lose net LP:

I was 40 LP plat 3 before I decided to play with some diamond friends. We get rekt in 3 games. I'm 0 LP. I play 1 game solo. I lose and get demoted to plat 4 75 LP.

To get back to plat 3, I need to win 4 games in a row. But that gets me to 0 LP.

It is not possible that my MMR is lower than what it should be for this division since I have a 50% win rate and am not division 5, and won before I played with my friends.

Where's the so called safety net?

All promos does for me is that it artificially forces me to lose LP if I maintain a 50% win rate. I not only don't climb, I FALL DOWN with a 50% win rate.