Nearly all of us here at DataStax have come out of the RDBMS world, have decades of experience with Oracle and other relational engines, and as such, we certainly respect the technology.
But the enterprises we engage with today use a combination of RDBMS and NoSQL, with the former mostly serving system of record use cases, while the latter is optimized for system of engagement applications (although NoSQL is making strong inroads into system of record apps as well). The fact is, today's Internet Enterprise applications necessitate a new database foundation that provides a more flexible data model (which goes beyond just support for unstructured data and JSON), a simpler and more cost effective performance/scale model, and an optimized data distribution model that provides both full write/read anywhere capabilities as well as always-on availability. And these are things that a NoSQL database like Cassandra was built to provide.
VP of Products, DataStax