sure some systems under correct operation will never ever be fully in sync as long as they are used
but most systems also do not need to be ever fully in sync
it's good enough that for a specific context they will be in sync in not to much time if that context stops changing
and that is something they do provide
e.g. after updating a JSON blob stored under a specific id that update will be eventually available in the not too distant future and if no future changes to that document happen then in the context of that document the system will be fully in sync. But because you have very man documents there will always be a document which isn't yet in sync and in turn the system as a whole will never be fully in sync. But in the end that doesn't really matter.
Also: no computer system will live that long, and if they aren't put in sync before they stop working they will literally never be fully in sync.
but a system as a whole never being fully in sync is a red herring argument which misses the point and focuses on an aspect which might sound like a problem but hardly ever is a problem at all
sure some systems under correct operation will never ever be fully in sync as long as they are used
but most systems also do not need to be ever fully in sync
it's good enough that for a specific context they will be in sync in not to much time if that context stops changing
and that is something they do provide
e.g. after updating a JSON blob stored under a specific id that update will be eventually available in the not too distant future and if no future changes to that document happen then in the context of that document the system will be fully in sync. But because you have very man documents there will always be a document which isn't yet in sync and in turn the system as a whole will never be fully in sync. But in the end that doesn't really matter.
Also: no computer system will live that long, and if they aren't put in sync before they stop working they will literally never be fully in sync.