Intel is planning for PCIe5 in 2021 (skipping on PCIe4), so AMD having gen 5 support in that generation would not be too unexpected. Though with the seperation of IO/compute dies I would not be suprised if PCIe5 would only appear in EPYC/Threadripper components.

I expect that the upcoming Ryzen socket refresh (AM5, which is expected to come with Zen 4/Ryzen 5xxx) to have support for (or even require) DDR5, as by then DDR5 will already be available for a year.

In the server market where the extra bandwidth is actually useful, the transition from PCIe 3 to PCIe 4 is pretty far along. Switches, NICs, SSDs and FPGAs supporting PCIe 4 are all either available or announced, and server-class GPUs should also show up next year. Most of those products have good reason to continue on to PCIe 5 as soon as suitable hosts are on the way.

There was less than two years between the release of the two standards, so two years between the release of supporting CPUs wouldn't be surprising.

Plenty of gen 4 SSDs on the market