Think about it, when you lose at 0 LP and you get demoted, you now have to win an average of 4 games to get back to where you were.
How does that make sense? 1 loss = 4 wins to get back?
Why not, if you're demoted, you just go into the previous tiers promos with a start of one loss in the promos, and if you win 2 games, you're back in.
You lost at least 4 games to be demoted though, since you have 3 games protection after promotion, plus losing at 0LP doesn't necessarily demote you unless your mmr is low already, which indicates you've been losing more often than expected to begin with and hence potentially belong in the lower division.
Having to play 3-4 more games to get back is you trying to prove you simply had an unlucky streak and actually do deserve that rank.
Usually takes more than 1 loss to get demoted. Also, you have to win 2 straight in order to win those promos; If you can't win those, you're back to 75 regardless, potentially with win-loss and a better streak to sap your future gains, and definitely having wasted time getting back down to 75.
Losing 1 game at 0 LP doesn't explicitly demote you. If it's so, your MMR somehow didn't prove you'd deserve the rank you achieved and considered it result of grinding rather than skill.
My mmr is ridiculously low, takes me 3 wins in a row to get back into promo, feelsbadmam.
Might work for plat+, but gold 1 and below get a free win when the re-enter a promo series, so it would effectively prevent demotion.
