The argument for shortening the Shadowlands expansions is more or less that Blizzard seems to be moving quite fast compared to previous expansions. We are already getting flying in 9.1, the landmass is not necessarily a new zone, but it is more than we used to get, not to mention that the raid is in the Maw, which seems quite early considering we would ususally have another raid before the final push.
Mostly though the argument lives and dies on whether we can expect 10.0 to be an expansion on a scale not seen before that manages ot overcome the hurdles of development that plagued Cata.
Though even with that it would be a tough sell. My guess is a normal patch cycle, but i guess we can only wait for tomorrow and see.