Can we talk about the promotional series changes?

So, Riot, I have to ask; why did you decide to change promotional series' the way you did? Instead of reworking a system that a lot of people seem to dislike, it's exactly the same as before in divisions of Platinum V + and basically a non-factor in divisions below Gold I.

Why is this system now completely different depending on what rank you are, and why did you decide to keep this system widely considered to be sub-par exactly the same for your most competitive players?

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Here's a system I've generally thought of as pretty simple and a decent temporary solution to promos:

Each time you win a game at 100 LP in a Division you get a stack, and each time you lose a game at 0 LP you lose a stack. If you hit 3/3 stacks you promote, if you hit -3/3 stacks you demote. Whenever you promote, you are 0 LP 0/3 in the higher Division. Whenever you demote, you are 100 LP 0/3 in the lower Division.

No promos, no need for sudden win-rate spikes, MMR and LP become directly connected, relatively lenient while also requiring a display of consistent skill the Division above. If you bump against the top a few times you promote, if you bump against the bottom a few times you demote. Simple.