When Apple switch to AMD, they could have the CPU+GPU+RAM(HBM) on a single interposer and cut the size of the logic board to 1/3rd it's current size. For the first time in over decade AMD is about be ahead of Intel in single core performance when Zen 2 (Ryzen 3000 series) comes out, and for the first time in 40 years AMD will have the process advantage.
2019 will just be an update to the current Macbook Pro is Apple follows tradition, with a redesign in 2020.