They've been playing the long game here...encroaching on your Drive storage for years
- Gmails didn't used to count towards storage
- Photos didn't used to count towards storage (unless stored in "Original" quality)
I'm wagering "Location Data" will be along soon. Then "number of passwords stored".
Next move will be "number of connected devices", even though that doesn't impact how much they have to store.
Finally they will get round to billing you on your number of individual body hairs (shaving in breach of T&Cs).