If they were recording all the necessary information there in the first place, anyway. The level of detail to replay or audit is usually higher than whats needed for direct operation, especially if some things go out through remote systems/accounts and back.

Sure, but you can have your append only layer be provided by a core banking system, and have your applications communicate with that using a higher-level protocol.

I don't see what would inherently disqualify Mongo from being the backing layer for that core banking system then.