Back to Zen3+, will it be AM4, AM5, or both? Given that AMD's using a chiplet design, in theory it wouldn't be too hard for them to make separate DDR4 and DDR5 I/O dies.
Those who know won't tell. I don't know, therefore I tell.
By the time Alder Lake arrives, AMD needs to have something new ready too. Without unlimited resources, they can't have a new socket, new chipset, new Zen, and new DDR and new PCIe controllers, all at the same time and properly tested. Without really unlimited resources, they can't steal 5nm wafer starts from under Apple's nose. So they decide on AM5 and DDR5, but old Zen (refreshed), old process (refreshed) and old PCIe, which is a good fit for a monolithic APU. The graphics will benefit a lot from DDR5 bandwidth here.
It will be something of a disappointment to enthusiasts, but so will be Alder Lake, so there. AMD then has enough time to finish Zen 4 and PCIe 5, and debug DDR5.
Regarding DDR5, there's surprisingly little information about it half a year before supposed launch, and the available info points at high latencies. I think that the technology is difficult like hell to implement and the first generation of controllers (from AMD, Intel, whoever) and modules will come loaded with bugs, with poor compatibility, stability and latencies. And gears, oh, gears! But you've got to start somewhere, and it better be a midrange product than something aimed at the most demanding crowd.