How placement works:
My season high at it's peak was 4400+ SR. I got there on the back of a VERY lucky win streak. I clearly do not belong there and should never place there of naturally reach there again unless I actually improve.
I'm consistently hovering around 4100-4200 NA/PC.
I one time dropped from 4100 SR to 3400 SR. Climbed to 3700 SR before the season ended. Won 2 out of 10 placements and still placed above 4000+ SR because that was where my MMR was at.
Why should you EVER place at your season high? That isn't your skill level nor is it based on your MMR which placements are based on.
All your idea does is give an illusionary sense of skill. And make people angry when their MMR keeps trying to drag them back down to where they belong.
That was what happened in the past.
The game used to give you like 40-50 games of bonus SR after placements to offset the fact that they placed you slightly below where you should be. It allowed players to climb very quickly while their MMR dragged behind. The problem was that they climbed too fast with the bonus SR and the game either starts pulling them back causing them to hard tank in SR after the 40-50 games. Or they just placed like 600-700 SR below from where they ended if they never calibrated back to where they belong before the season ended.
All this did was undo the progress many players made despite that progress being entirely smoke and mirrors.
https://us.battle.net/forums/en/overwatch/topic/20753625906#post-14