The smartphone is the messaging centre where we coordinate our electronically mediated communication. Whether you get 7 notifications from different 7 apps or 7 notifications from 1 app should not make a difference from the point of view of attention spent, focus lost, time wasted.
The following communication pattern has emerged for me, without any design on my side:
Snap: frivolous and fun
Whatsapp: family, European friends, the Toyota/VW of messaging
LINE: Asian friends
Skype: high priority work stuff
Email: low priority work stuff
Insta: friends boasting about their life ;)
In other words, different apps function as a way of categorising messages. I find this siloing convenient. Siloing reduces the risk of sharing the wrong kind of message with the wrong kind of person, which has been a great problem with unified messaging systems (e.g. Facebook, or Google's centralisation of all accounts when they introduced G+).
As these different messaging systems are (simplifying a bit) fungible, and it is near frictionless to switch between them on a modern smartphone, the phone (it's number or IMEI) is the new dominant form of electronically addressable social identity. This should worry the Facebooks, Snapchats, Whatsapps etc of this world, because the network effects which is the driving force behind their business models has been weakened to a considerable degree.
In summary: I welcome the widespread use of heterogeneous messaging and notification apps on the phone, it makes my life easier and safer.