What if two users want to change e.g. the telephone number of an existing record during a network partition.
There just is no obvious way to merge a telephone number. One of them is correct, the other is incorrect.
Can CRDTs solve my simple problem?
In your example, that might mean changing the single "phone number" field to many "phone numbers", so that merging the two writes results in a customer record with two phone numbers. This preserves the data, but pushes conflict resolution (which number should be used?) out into the consumers.
i.e. this is solved with CQRS and Event Sourcing and it probably works in like all databases. It's quite complex but pretty reliable, I'm pretty sure that everybody already built at least a extremly simple append only event log.
Basically though that makes a CP tradeoff, since a network partition that hides the primary kafka replica from some of the writers causes writes to fail.
There’s simply no way to have a globally distributed system that’s always available and always consistent, either you get inconsistency or you drop writes.
where you store (i.e. which database system, cluster whatever) your stuff doesn't matter.
all CRDT will give you a deterministic outcome and provide enough information to allow a user to decide if this outcome is acceptable