1) imagine having to sit through the first few weeks of comp facing high tier players way above your skill rank till they eventually climbed back. Every low tier player would get screwed in a ripple effect.
2) This is a real slap in the face for players that have already been maintaining their for seasons. If they didn't deserve their rank, they would have dropped anyway
3) placements already do this in a way by re-assessing rank every season. They just use your last season SR as the base point for your matches (if it's a new account, starting base is Gold)
5) Go buy a GM level account today which is still on placements. Do your placements. If you're a plat player, see what rank you get. (Spoiler alert: it will still be plat.)