Express is actively maintained and has been relatively API stable since its release in 2010. It remains a solid choice for middle-teir work.
Every software release is an experiment, and sometimes those experiments should be scrapped when something better comes along. Perhaps Polymer is a good example of that.
Sometimes when better ideas come along, existing ideas can be adapted (with breaking changes) to keep the good ideas from the existing framework while responding to change. (Angular 2 reacting to React).