I guess on Jeremy Ashkenas twitter feed he called Backbone "finished" in terms of API and feature set after the 1.0 version. When you look at the contributions graph, its practically non-existent: https://github.com/jashkenas/backbone/graphs/contributors
Add in the Google trends graph compared to Angular and React: https://www.google.com/trends/explore?q=backbone%20js,angula...
Then when you look at the other libraries in the ecosystem, its like a ghost town.
backbone.forms - Open Issues: 172, Issues closed in 2016: 14, Last Release: Jan 21, 2016
backbone.localstorage - Open Issues: 45, Issues closed in 2016: 1, Last Release: Jan 22 2015
backbone.paginator - Open Issues: 20, Issues closed in 2016: 7+, Last Release: Feb 3 2016
backbone.validation -Open Issues: 79, Issues closed in 2016: 1, Last Release: May 8 2015
So not only is interest waning, the creator has called the library "finished" with not much more in terms of features coming and the ecosystem surrounding Backbone has all but dried up. I would be hard pressed to say it still has a bright future.
It may be less flavour of the month, but that seems par for the course with JS frameworks.