npm is pretty much the prime use-case for CouchDB. REST API out of the box, replication is a core feature and not just a scaling feature (multi-master, MVCC) and the validation/access control is pretty much made for it. The npm registry is implemented as a CouchApp for a reason.

I agree that from a functionality perspective this is largely true, at least on paper. That said, if running costs become a significant bottleneck CouchDB becomes a less obvious choice. Sometimes there's a reluctance to migrate from one technology to another as requirements change over time but this seems one of those occasions where exactly that step is required. Sometimes it's good to take a step back, look at your current requirements in terms of cost and performance and determine what technology best suits your needs. I would question the know-how and objectivity of anyone that would land on CouchDB/node.js in this instance.

There's no node involved in the registry itself. This is purely CouchDB.

CouchDB has better replication support (e.g. multi-master, MVCC).

a lot of the central node community started on couch

Why?