The use case is local user DuckDB talking to MotherDuck for $.

This is not commercially a terrible idea. Why keep paying Snowflake for bog-standard SQL query workload when SF makes it easy to migrate to Iceberg & commodity engines like MotherDuck?

"moving data around" is what millions of people of do all day, every day.

uh, doing analytics type queries on large datasets that postgres would choke on, as an RPC? I'm using it (ducklake specifically) to build a lakehouse RPC server that can scale horizontally based on resource utilization in k8s.