For which scenarios would it make sense to still account for "setting up replicas within a single cluster" as compared to "CCR" - given the constraint that "one is limited to a single data center". i.e. we are not making a case for cross datacenter replication nor index locality wherein CCR seems like a good solution option to consider.
In my opinion, when a cluster has non-data nodes (i.e. master nodes, ingest nodes etc), accounting for replicas could still make sense, in order to provide for fault tolerance within a single cluster since setting up CCR would mean more hardware (as compared to the hardware needed for replicas alone).