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.