Joe Rossignol, writing for MacRumors:
Kuo expects Apple to retain the Lightning port given it has a slightly slimmer design compared to a USB-C port, to sustain MFi Program licensing income from Lightning accessories, and because he believes USB-C’s high-speed data transmission is “still a niche application” for iPhone.
The 12.9-inch iPad Pro already supports USB 3.0 speeds and fast charging with a Lightning to USB-C cable, while iPhones and other devices with a Lightning connector still transfer at USB 2.0 speeds.
This should allow for backwards compatibility with existing cables and accessories while improving charging time and data transfers when using a Lightning to USB-C cable.
I think Nick Heer had the best take on this:
One of the great things about the Lightning connector is how it’s able to abstract all of this under-the-hood stuff and make it really consumer-friendly. […] Lightning is just Lightning, even when it isn’t; the only thing consumers will notice about the new iPhones will be how much faster they charge.