Those are streaming/pubsub services though, this actually claims to be a store. I feel that's an important difference.

Do people just point their system journal at Kafka and wait for something to break?

At my previous job we built something similar to this out of rabbitmq and mongodb. I always wondered what the other big log companies used. Mongodb seemed like a pretty good fit, but a pure append only database might be even better. Trimming performance in MongoDB was subpar so we worked around it by creating a new collection for each day, trimming became a simple operation of dropping a collection at the end of each day.