Interesting observation I think there is a need for Facebook to examine what is unique for each platform. Though I would think that the instagram stories are well done for a clone feature but in Whatsapp it seems not so useful.
There is a need to balance business needs and user needs with new features. It seems like a trend to make stories disappear for now cause that is what may potentially make money but it may not work out for users. There could have been some other way to introduce the new feature over a period of time, but it depends on the platform audience.
For notification I guessing for now the user has to turn it off at the OS Level, to not get pingged.
But we will never know the true effects and results of the changes to these platforms as they have the data.