Checksumming at the file system level solves the problem of corruption that occurs off the media (on the bus). The media has checksumming on it that allows detection and recovery from errors that occur on the media itself.
The question I'm sure Apple engineers have were - How often do we see BitRot occurring off media, and is the media that we're deploying sufficiently resistant to bit-rot?
And, with APFS's flexible structure, this is a feature that can be added at a later time. Probably made sense to deliver in 2017 something that was rock solid that they could build on, than to either (A) push out the delivery date, or (B) not fully bake all features of the file system.
The question I'm sure Apple engineers have were - How often do we see BitRot occurring off media, and is the media that we're deploying sufficiently resistant to bit-rot?
And, with APFS's flexible structure, this is a feature that can be added at a later time. Probably made sense to deliver in 2017 something that was rock solid that they could build on, than to either (A) push out the delivery date, or (B) not fully bake all features of the file system.