Backbone doesn't become any less great because a similar project has appeared. If we are to believe the claims of the creator, Spine is fundamentally different from Backbone, so I dont think a fork would have sufficed.
By introducing a too-similar competitor, (in general, not necessarily Spine) you risk fragmenting the above, which leads to many okay projects, not one great projects.
It's unreasonable to suggest that anyone who has an idea for a project simply abandon it because someone's already working on something similar. It's also unreasonable to suggest that if they do end up writing something really cool that touches on a few concepts of an existing project, that they should keep it to themselves.