Agreed that such setups can be simpler than multi-master replication. Yet, there's still a hard distributed systems challenge even in the active/passive setup like you describe. When a failure occurs, the clients need to agree which host is the master and which host is the slave. And in the presence of network partitions, it can be impossible to say if the old master is dead, or partitioned away, and hence still happily taking writes.

The most common way in which such systems failover is by fencing off the old master prior to failover. Which in itself can be a hard distributed problem.