Open-source counters most of the issues around monoculture. Remeber that the first browser wars occurred between two closed source products.
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.
You're basically saying there should only be one web browser and no one should try alternate approaches to common problems unless they're starting with the same codebase.
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.
How does open-source counter the issue of monoculture? If there is a monoculture and it's open source it still has the same problems. You can fork but there's a monoculture so your fork is irrelevant. You can submit a patch but there's a monoculture so your patch doesn't get accepted.
Open source isn't a magic bullet, try forking Chrome and see how far you get without prominent adverts on the most popular page on the internet and investing millions into packaging your browser as part of Flash/Java/etc. updates.