Your first paragraph is, bluntly, incorrect. Cassandra guarantees that data will always become consistent. This is automatic [1] for normal operation, including in the face of temporary failures. Permanent failures require running a "repair" process to rebuild the failed machine from other replicas [2].
I think you've also misunderstood how quorum works; it is a quorum of the replica count, which tends to stay constant over cluster lifetime, not machine count.
You are right that the current counters are an afterthought. I linked in my concluding paragraph, where I talk about improvements for Cassandra, "A new design for distributed counters." [3]
[1] http://www.datastax.com/dev/blog/modern-hinted-handoff [2] http://www.datastax.com/docs/1.2/operations/node_repair [3] https://issues.apache.org/jira/browse/CASSANDRA-4775