Actually this is incredibly easy to answer. There are a few very real issues that causes people to completely reject the Mac Book:
1. The CPU is crap in the machine, anybody doing demanding work will suffer through very sluggish behavior.
2. The Single USB-C port is just a bad idea for a MBA replacement. It might make sense on a netbook (pretty much what a Mac Book is) but it sucks for a general purpose machine. By the way I fully support the move to USB-C, it it set single port that I have trouble with.
3. That current USB-C port doesn't support TB3. This means docking stations suffer performance wise as you aren't getting any high speed goodness out of that port. An upgrade here makes for a more interesting Mac Book for some users.
4. That CPU thermally throttles pretty badly at times.
5. While nice the screen is small. I'd rather see the MBA's upgraded to retina screens. For my usage even a 13" screen is crowded.
That covers many of the issues but don't take this as knocking the Mac Book, for some users it is a good machine. However a lot of users out there simply need more from their machines. MBA is actually a pretty good value (if it is possible for Apple to have value in its products). On the flip side Mac Book is a poor value, it costs a lot and performs poorly. Like others though I think Apple can recover value in Mac Book with some solid upgrades which frankly wait on Intel to deliver.