Lets view the possible scenarios:
1) "Vanilla as it was": if people dont like it means Blizzard was right "you think you do but you dont"
2) "Vanilla with QoL changes": If people dont like it people will say "its Blizzards fault for not delivering the ACTUAL Vanilla"
So as you can see...is better to deliver "Vanilla as it was"
Worst case scenario "Blizzard was right in the first place" which is much better than "Oops we messed up, sorry"