AMD in a blog post today has confirmed that AM4 will support the upcoming Zen 3 architecture but only the X570 and B550. AMD stated that the reason for this is because the limitation of the flash storage chips that store the bios. The bios storage chip has already been an issue in the past. An issue you could argue they should have resolved sooner.
It is possible that given the fact that the only reason the other chipsets aren’t supported is the bios storage chip, that in the future AMD allows you to update your bios to support Zen 3. It’s possible but whether it happens in the future or not is anyone’s guess. AMD has relaxed the reins in the past for its customers so I guess we will wait and see.
I would think it wouldn’t be a great move for AMD to sell the X570 and B550 motherboards and then move to a new platform next year giving those who purchased an X570 and B550 motherboard no upgrade path. So how long will AM4 be supported? At this time AMD says that all depends on I/O technologies as such changes usually require a different pin count or layout.