James, I had not considered the model of one ES instance per service. I understood that the point of Event Store was to serve two purposes:
1. Be a permanent audit record of domain events, replayable on demand.
2. Be a bus by which to inform other services of domain events.
In other words, to be an integration layer between services instead of belonging to one service only.
Are other folks doing event sourced microservices out there? Are you using ES per service or one global instance as an integration layer?