Sure, let me know if I'm following this train of thought by drawing a parallel.

If we swapped out rendering engine with game engines. It would be best if we kept to a few game engines and focused on the game mechanics and story?

In that spirit, I would agree with you. Much like the examples you provided, its more about who or what controls the full stack of experience. It's just, quickly thinking about this I'm struggling to find a compelling reason to use a browser beyond the basics. Since the core features I seem to require are satisfied in any browser that isn't provided by an entity that puts capital interests before the user too harshly. Plus the addition of an adblocker and custom theming.

Ultimately, it just needs to show the webpage safely and precisely how it was intended to be seen, without ads. Through the support of extensions, I suspect that would satisfy any additional requirement someone could desire or imagine without the need to delve much deeper into custom browsers. At least, a browser for general use without a specific purpose. But perhaps I'm misjudging the capacity of those potential extensions in the face of a customized browser?

I suspect, how opera paints a bunch of features down the left side may be hard to replicate on another?