I'm not really familiar with the ruby development process and community and haven't used the language much, but I could imagine the MRI developers spend considerable time advancing the actual design of the language (syntax, standard APIs, etc.) whereas the other Ruby implementations "only" need to implement that design and thus can maybe focus more on performance. If MRI were to die, that design work would still need to be done in order to push the language forward. As such it could be seen as a symbiotic relationship, not competition.
Yes, I'd say that's at least partially true. The innovation on JRuby's side is in new ways to integrate with the JVM, rather than in API or language design. MRI folks need to innovate both in how they design the language and APIs and in how they implement them.
