Future Mac OS support for hardware is interesting, in the grand scheme of things 2015 to 2017 Macbook Air's at least are pretty much the same thing. In future the OS could go the route of having iOS chips as well as Intel, ARM only, or they just bake it into existing hardware and restrict features to devices with say 8GB RAM or more.
Personally I'm not sure buying a Mac in 2018 you'll get the same decade of support that people have got used to since they shifted to Intel. Mac OS is lacking the Home app, Watch app, Swift playgrounds - which are core to Apple's future of - Home automation, wearables and coding. Then combine that with iPhones X's and iPad Pro's benching 10,000 and Macbook 12'' only pulling in 6,000, even Macbook Pro's bringing in 8,500!
It really wouldn't surprise me in 2019 to see Mac hardware running fully on ARM with a MAC OS alot more iOS like and running iOS apps, the existing hardware just wouldn't get the iOS apps but would run existing Mac Applications just like they do now.