Nomenclature aside . . . and I am not absolving Apple of this. For whatever reason, they have done some weird things with regards to naming. And they are not consistent across much of their product line . . . not that many manufacturers are . . . I need to understand marketing better, no doubt . . .
That aside, there are really are some important differences between the 2017 Macbook and the just announced 2018 Macbook Air. If I quickly compare the two and look at the benefits:
Macbook:
-Cheaper (assuming you standardize storage at 256GB/512GB) by $100
-Noticeably and significantly, for me, smaller/thinner/lighter
-Silent (it is not 100% clear at this point, it seems, if the Macbook Air has fans . . .Apple is using a cpu that needs no fan, but it is unless if they chose to go fanless . . . maybe they need extra cooling for a TB3 controller, for example)
Macbook Air:
-More storage options (in addition to the two middle options above, you get 128GB, 1TB and 1.5TB)
-Two TB3 ports (Macbook does not even have one; it is stuck with only a USB 3.1/3.0 Gen1 port)
-Can drive a 5k external monitor
-More modern cpu likely allows for better hardware decoding support for the most modern codecs
I think they are just different . . . that it is hard to really say one is generally better than the other, unless one of these things is really important to you.
I would preferred that they had eliminated the 2017 Macbook Pro without TB (which they seem to have done), and introduce two new Macbooks . . . whatever they name:
-One 12" and one 13.3" or whatever
-Same internal options . . . just the screen and battery being different
-Included all the storage options of the new Air
-Included the modern dual core cpus, but definitely fanless
Basically, do the same thing they did for the iPhone Xs and Xs Max . . . the only different is the screen size and battery. That would give people two sizes for the Macbook and two for the Macbook Pro. Apple repeatedly said the Air was the most beloved Mac, so I guess the name and shape is really important to many people.
I would also have transitioned the whole Macbook (cheaper) line to A12 chips, but Apple really seems to be going with the iPad Pro with Keyboard instead.