Furthermore...
On June 5th, 2018, the PCI SIG released version 0.7 of the PCIe 5.0 specification to its members.
PLDA announced the availability of their XpressRICH5 PCIe 5.0 Controller IP based on draft 0.7 of the PCIe 5.0 specification on the same day
https://www.plda.com/products/xpressrich5
and...
Historically, the earliest adopters of a new PCIe specification generally begin designing with the Draft 0.5 as they can confidently build up their application logic around the new bandwidth definition and often even start developing for any new protocol features. At the Draft 0.5 stage, however, there is still a strong likelihood of changes in the actual PCIe protocol layer implementation, so designers responsible for developing these blocks internally may be more hesitant to begin work than those using interface IP from external sources.
From here: https://en.wikipedia.org/wiki/PCI_Express
AMD had plenty of time to include PCIe 5.0 even in the Draft 0.7 stage which is pretty much the same as final draft/release, but decided not to.
No possible way that could backfire or be impossible
... Just how long do you think it takes to make that kind of design changes? This requires a major change in silicon. It would have had to been done more than a year ago.
Zen 2 will be the first CPUs supporting 4.0.