So much this.
I work on Flow [0] at Estuary [1]; you might be interested in what we're doing. It offers millisecond latency continuous datasets that are _also_ plain JSON files in cloud storage -- a real-time data lake -- which can be declaratively transformed and materialized into other systems (RDBMS, key/value, even pub/sub) using precise, continuous updates. It's the materialized view model you articulate, and Flow is in essence composing data lake-ing with continuous map/combine/reduce to make it happen.
I was asked the other day if Flow "is a database" by someone who only wanted a 2-3 sentence answer, and I waffled badly. The very nature of "databases" are so fuzzy today. They're increasingly unboxed across the Cambrian explosion of storage, query, and compute options now available to us. S3 and friends for primary storage; on-demand MPP compute for query and transformation; wide varieties of RDBMSs, key/value stores, OLAP systems, even pub/sub as specialized indexes for materializations. Flow's goal, in this worldview, is to be a hypervisor and orchestrator for this evolving cloud database. Not a punchy elevator pitch, but there it is.
[0] https://estuary.readthedocs.io/en/latest/README.html [1] https://estuary.dev/