its a commercial decision
At first they said no to 300 series, to make people buy new motherboard. Now AM4 is at its end and they want to clear inventory so they cut cpu price and they make it compatible on 300 series boards
Wrong.
They still cant do proper support on boards that dont have more than 16mb bios size. They have to use bios with dropped support for certain cpus etc to get latest cpus to work. So it was not a shady commercial decision but a technically limited one.