To add to what Armon said: we always had Consul in mind when building Serf. But Serf was a necessary building block on the way to Consul: highly available, lightweight membership management. Every distributed system has a membership problem, and we didn't want to reinvent the wheel for every system we built. Serf provided an incredibly stable and well proven foundation for us to build Consul on top, as well as future products that are already in the works...
And while you may not see Serf as having much use, we've personally helped and seen Serf clusters with many thousands of nodes. Serf is very useful to these organizations for its purpose. And while some of these orgs are now looking at Consul, many don't need Consul in the same way (but may deploy it separately).
We're not stopping with Consul. We have something more on the way. But we now have some great building blocks and experience building distributed systems to keep doing it correctly without having to rebuild everything from scratch.
And while you may not see Serf as having much use, we've personally helped and seen Serf clusters with many thousands of nodes. Serf is very useful to these organizations for its purpose. And while some of these orgs are now looking at Consul, many don't need Consul in the same way (but may deploy it separately).
We're not stopping with Consul. We have something more on the way. But we now have some great building blocks and experience building distributed systems to keep doing it correctly without having to rebuild everything from scratch.