"It does make me hesitate to recommend Swarm for a new service, with the risk that it will be end-of-lifed soon."

This was my exact concern. With the announcement at the end of 2017 that Docker EE would be integrating Kubernetes, it gave me pause as to whether to put Swarm into production now, when there is the possibility Docker could be moving away from Swarm entirely in the near future.

Do you have any sources that there will be continued investment in Swarm?

All recent blog posts I see are hyping of the features of the Kubernetes integration.