Sorry, I don't know how I missed day zero-day.. Anyway my point still stays..
> At the end of the day, the engine of the major browser engines is open-source anyway.
Open source is not enough. The question is: who controls it? AOSP is open source, Chromium is open source. But Google controls both. It means that Google can push for what is good for Google... even if it is bad for the user. E.g. preventing users from blocking ads. Not that it does not have to be with evil purposes (though Google has been shown to be evil enough already): it's enough for Google not to care about something for it to impact Chromium/AOSP.
That's the whole point of competition: you want the users to have choice, so that it pushes the companies towards building a better product. Monopolies never serve the users.
Now you say: "ok but it's open source, so if you're not happy you can fork away!" -> which precisely brings us to two browsers, like now with Chromium and Firefox.
Yes, Chromium and Blink are open source, but they are effectively Googles open source project. If you're unhappy with their direction you'll need to fork it.
Running a different compiler is also useful to find bugs in your project, and in the compiler itself. I would imagine this applies to the web just as well — a web browser implements an open spec (just like a C++ compiler), at the same time being much more complicated than a C++ compiler.