RethinkDB could definitely handle your load.. the advantage over CouchDB being you can do a bit more than just your value, and get indexing on other fields in your data structures. At your data load and growth, I'd be surprised if a small cluster couldn't handle your load for 8-10 years without issue... Since you're using JSON structures being able to query your data beyond keys is really nice... A document database is probably a more flexible option than a straight kv store.

If you wanted to stay closer to key/value with nice clustering and scaling options, I might suggest cassandra. To be honest, that would be overkill as a relatively small RethinkDB cluster would likely handle your load for several years without issue.