Wouldn't it be more damaging if the authorities are able to sync and recover the chat logs (they had time wipe the logs)?

If they are able to take the journalist's sim card which is linked to their Signal account and then are able to recover the chat logs the journalist would be done for.

Of course the supposed journalist we're speaking of is already in a bad spot if they're interred. However, they might have plausible deniability with respect to their phone if there's no compromising chat logs to recover.

To your point about exporting, it would be nice. Ultimately, why can't we have both worlds by way of toggling the function?