Neither new variants and boosters are reasons for revokation as both are better served by information about the vaccine administered and a timestamp.

if there is a new vaccine required or some follow up procedure or whatever it would make more sense to hand out a new certificate that validates this fact instead of revoking the old one that is actually still perfectly able to validate your previous actions. In fact it might be very beneficial to keep them around to give approval to another step like lets say you will only get that booster shot after you are able to present validation of your vaccination or similar... invalidating these certificates by giving them a date until they are valid is very easy without explicitly revoking certificates.

At least here around I never read that argument. Given that it's initially valid for only 6 month I don't think that's really an issue.