"Here's my complete line of reasoning: You can build mutable systems on top of immutable (append-only) systems. But is that a good idea? Yes, it is, for systems which don't have huge amounts of (non-static) data, and/or system which need an audit-trail anyway. And these are more systems than one may initially think."
I disagree that immutability is a negatively defining factor here re. data size or capabilities of the database.
If you look how many Big Data systems process data, you'll find that at the core of many, is an append-only log. For example: Kafka is a log (https://engineering.linkedin.com/distributed-systems/log-wha...), and looking at Apache Samza's architecture, we can see how a log is at the core of it (https://www.confluent.io/blog/turning-the-database-inside-ou...). In less Big Data orientated databases, there's always a log of operations (sometimes also called a transaction log or replication log) to keep the track of changes.