In a new video, LinusTechTips reveals AMD has committed itself to ensuring that Socket AM4 will stay around until 2020. AMD is not planning to replace the socket until DDR5 and PCI Express 4.0 are available.
So for the next couple of years, enthusiast will be able to upgrade to new CPUs and continue to use the same Socket AM4 motherboard. AMD promises it will not change any electrical requirements so after a BIOS update you should be ready to go.