Clustering columns do not make Cassandra updates any safer: they only reduce the scope of conflicts. It's still last-write-wins--the approach that Cassandra's own blog recognizes as "a high potential for data loss".

That's why Cassandra offers choice at a query level.

It doesn't give you a choice: you get last-write-wins, or some limited merge functions with CQL types. You can't get generalized CRDTs in Cassandra, because they won't expose conflicts to the user layer. Cassandra gives up on a whole class of safe AP datatypes as a consequence of this restriction.

Now you are arguing about a lack of a particular builtin feature. Riak is last-write-wins by default with optional vector clocks and builtin CRDTs. Cassandra is last-write-wins by default with clustering columns allowing to implement an equivalent of Riak vector clocks and CRDTs in the application. If you include a client id in the primary key and use client-side timestamps, you essentially are doing vector clocks and there are no conflicts guaranteed - users of Cassandra have been doing it for years.

> they won't expose conflicts to the user layer.

They will, you'll see them as separate rows of the same partition, and then you can merge them as you wish in the application.