Plainly stated, AMD did the best they could to keep their promise. That said, they have done a HECK of a lot better than Intel. 3 generations on the majority of boards is nothing to sneeze at. It does seem highly unlikely that someone will buy a Ryzen 5 3600 to drop into an A320 board. The good news is that most people will be just fine as long as manufacturers play ball, though honestly it always comes down to manufacturer support. AMD did their best to offer it, it is up to the motherboard partners to implement it. Not exactly AMD's fault. Lest we forget Intel's shenanigans with the i9 CPUs using the same socket as the i7 8th gen, but needing a different chipset, making what should be a drop in upgrade a motherboard replacement.
As for future CPUs and sockets, I'm expecting Zen 2+ to be on a new socket, AM4+ or AM5. This will allow them to move to DDR5, provide better PCI-E 4.0 support, and optimize power delivery for revised CPUs.