It is very unlikely Apple will adopt this approach because while solving a problem, this creates another two:
1 - with double notification for recent and old messages, this brings a significant visual load. Imagine many apps having double notifications (even thought this double notification is supposed to be only a transition to the “+ number”);
2 - it will be hard to keep track of an app on the springboard while it’s changing its appearance, even so slightly once in a while (even though many users are typing the name of the app instead of identifying it visually).

 

But the good thing is, Apple can keep its icon as it is right now and allow users to adopt "+ <number>" count.