Imagine Pepsi deciding that they are done with Pepsi Max, arguably their biggest brand, after Pepsi itself, and decides that it's now Pepsi Cake. Just kill of all references to their biggest brand. That wouldn't go down well and Microsoft is only getting away with it because pretty much everyone who needs it already have their subscription.
As an IT person I can guarantee this rebranding is going to cause confusion that will waste my time.
That being said, Coke killed Coke Zero and seems to be doing okay I guess ¯\_(ツ)_/¯
The common denominator is ego. Office may be a globally recognized brand, but it was defined by someone else, and the person making the decision needs to mark their territory.