If they go this direction, I see the following happening:
1) It will be an ARM-based processor. Macbook will have an ipad ARM processor and Macbook Pro will have an actively cooled 15 watt version of generally the same ARM processor.
2) In the first year, Macbook will be ARM only. Macbook Pro will have an intel processor that it will switch on when running x86 apps kind of like how a GPU turns on when using certain programs. I only see this sticking around for a gen or two.
3) Cost will not decrease, except on the very low-end
4) MacOS will just be the same code-base as iOS except a UI layer with mouse/keyboard support. Apps will run on either one seamlessly except the UI will be changed.
5) Every legacy program will be "broken" and have to be reprogrammed for ARM compatibility. Many programs will never be reprogrammed, mostly games and niche apps. Many of the programs that are reprogrammed will not be as fully featured as the ones lost, possibly for years as they catch up.
6) Apple will lock down the ecosystem just like iOS. All app installs must be through the store, and apple takes their 30% cut. Some applications will never be approved that people may otherwise want to install.
7) iPhones still will not have USB C. iMacs will become docking stations for iPhones to turn into a desktop environment and run the keyboard, mouse, and monitor.