In fact, I think that a single OSS project is far more effecient than attempts at standardization across competing products when it comes to user-beneficial innovation.
Look at the open-source UNIXes. Nearly all the value-add has come from cross-pollination of "proprietary" and not-yet-standardized enhancements, which are consumed by users and application vendors targeting those platforms.
Being open source doesn't change the fact that it's the same codebase.
Why should there only be one browser engine? I'm a web developer and I hate cross-browser testing/compatibility, I prefer to use Chrome for it's devtools. I would hate for there to only be one browser in the world.
Duplication is not equivalent to standardization.
On the mobile space users are at mercy whatever WebKit version gets to be integrated into a specific OS release.