One of the features I noticed from rbenv is that it does less than RVM. This is a good thing, but proposing this improvement to RVM would essentially be asking the maintainer to remove features.
There are many projects with widespread use that have become core projects in the Ruby community (Rails, bundler, RVM, etc). It's reasonable to expect that issues or design decisions get discussed on the project mailing list before you try and divide the community.
In any case, to defend against claims like yours (which are common, it seems), it does seem wise for anyone wishing to create something new to post to existing mailing lists with their ideas. Once the maintainers say "No" to your idea to rip out 90% of their system, you can then create your new project and have the valuable mailing list link to say "Look, I tried."