Arguably, this problem happened to Java web frameworks.
Too few and you lack innovation and fresh ideas, and things get driven to ideological extremes that aren't healthy or suitable for everyone (I don't want to start a flame war, but I feel a bit that way about Rails).
Too many and none of them get enough community or support to reach critical mass. What this meant for Java is that most people ended up being forced to use J2EE frameworks designed by committee which were awful, and drove a lot of people away from Java altogether.
Actually what I've seen with Java is that the committee-designed J2EE frameworks failed to keep up and got left behind. In the last five years, the only times I've heard of teams using EJB or JSF were either moving away from them or embarrassed by them.
More number of js frameworks there are for serving similar needs, more fragmentation there will be. Fragmentation leads to in stagnation in development process (slower bug fixes and feature delivery) and broken community support that can't gain critical mass.
Ruby community somehow avoided this fate, and almost everyone pushed for Ruby on Rails as the single dominant web framework of choice for Ruby. Sure there's Sinatra, but it's miniscule compared to Ruby.
> Ruby community somehow avoided this fate, and almost everyone pushed for Ruby on Rails as the single dominant web framework of choice for Ruby.
Well, that's not really true. There was considerable pushback against RoR as a web framework, leading to lots of alternatives, most notably Merb, and to Rack as an underlying platform for web frameworks.
Eventually, though, Merb and Rails merged, Rails incorporated Rack, and Rails incorporated a mechanism for running other Rack applications as endpoints alongside traditional Rails endpoints. Furthermore, while Rails is the dominant full-stack framework (largely, through having directly incorporated much of the competition into Rails), there's plenty of alternatives for many of Rails core components (e.g., lots of alternative ORMs to ActiveRecord.)
