Why does a fully event-sourced CQRS implementation with separate databases justify its added complexity?

This flavor is the only one that provides strict per-aggregate event ordering, which closes the silent out-of-order corruption risk present when separate databases are kept in sync without event sourcing. It also uniquely enables reliable replay of history to rebuild a read model from scratch, and genuinely independent scaling and technology choice for the write and read sides, since they're fully separate systems connected only by an event stream. For a system with real regulatory, auditing, or high-throughput scaling needs, such as a bank's account and loan services, these specific capabilities are worth the added operational cost of running a dedicated event store and a framework like Axon to manage it. For a system without those needs, a simpler flavor is a perfectly reasonable choice instead.