Earlier quoted context omitted.
Yeah... but I kinda wouldn't mind if they were all using the same open source rendering engine under the hood.
... But that's exactly what they're saying we DON'T want. A lot of browsers re-used IE 6's rendering engine because it had a monopoly. That only made the situation worse. A browser's rendering engine is the ONLY part that matters, and if anyone has a monopoly that is "bad" because competition helps innovation. Each rendering engine winds up, for good and bad, with a small group of people deciding what features do and…
I don't necessarily agree with your assertion that a browser's rendering engine is the only part that matters. Internet explorer ships with windows by default. Mozilla (firefox) has policies that focus on privacy. Chrome focuses on speed, and, well, serving Google.
These are all things that differentiate the products in the market and aren't necessarily rendering engine related. Yes - if one entity maintained absolute control of the rendering engine, then the needs of each vendor would not be served - but then it wouldn't be open source, either.