Yeah Apple's laptop lineup is a mess (the iPad lineup is pretty bad also and it looks like it will get more confusing next year. Are we moving to an iPad Pro only lineup).
What they should have is:
Low end MacBook: 12" retina screen, powered by some variation of the A10X chip that is going in next years iPads, 1 USB-C. This would bridge the gap between iPads and Macs and would be great for students who don't need a bunch of power. If they price it around $700 it could be the perfect entry level Mac. This would replace the Air.
Mid tier MacBook: 13" retina screen, powered by intel, 2 USB-C.
High end MacBook Pro: 13" and 15" retina screen with wide color gamut (maybe HDR), fast chips from intel, 4 USB-C, OLED touch bar.
Something for every price range. That should fix the low Mac sales.