Now imagine you are a returning player for shadowlands.
You are not going to pick WoD to level from 10-50 because there is no flying but you have flying in Cata, MoP, BC and WotLK.
A returning player would like to try BFA. WoD or Legion as it would be a new experience. But forcing them to have a new experience on the ground from 10-50 is going to be a major turn off.
Blizzard shot themselves in the foot when they committed to the patchfinder shenanigans.