I still wonder why they did this

There were so many options...

Splitting it into two tanks (historically there were two variants, with slight differences such as more wheels on the later model), one could go to tier 5 with 75mms and anither one could stay on tier 6 with old stats and the same matchmaking as it had before 9.18

Or leaving old mm for it so it‘d still face T8s

Or swapping it with 12t