2020 seems a bit close. Especially considering an all new, redesigned Mac Pro is coming next year. Seems odd to put all of that effort into a new Mac Pro only to begin switching away from x86 a year later. On that note, while the ARM chips Apple are putting out compete with mobile chips, they don't yet compete with mid or high-end desktop chips like those in high end Mac mini's, the iMac Pro, and presumably Mac Pro. I don't see Apple maintaining an x86 and ARM version of macOS.
I guess they could move the lower end Macs over to ARM and have a forked macOS, but even that seems a bit of an odd move to add yet another operating system. I am not saying Apple does not move to ARM, but I do think 2020 seems a bit ambitious and does not feel right. That is just about a year off (year and a half if we assume WWDC 2020), and I just don't know that we are seeing signs that something like that is coming.
I will say that it is rather interesting that both the MacBook and the iMac were not updated this year. Those would be the machines I would guess would be the first to switch. However, as I said, what would they do for the rest? I don't think ARM is fast enough to replace i5 and faster systems. Maybe I am wrong about that?