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.