For me, a big selling point for the PEP is that it is not a big change – it only skips 11 version numbers.
It doesn’t solve all issues that a completely new versioning scheme could theoretically address, but that’s OK.
(FWIW, if we did adopt four-digit year version component, I’d be very tempted to abbreviate it anyway.)
What would skipping 11 versions break? All I can think of is planned changes (like deprecations) that would need to be renumbered, across the ecosystem. Thankfully, unless a change is planned a decade or more in advance, it’ll be clear if the number is old or new.
If CalVer is adopted, and later we need to switch away from it (e.g. we want to do more feature releases per year), we can skip enough releases so the that number doesn’t look like a year any more, and treat the number as a counter again.