> Do they want to take action to address it?
Within the constraints of the assumptions I've outlined, yes, and they have.
One example is the fight for asm.js/Webassembly against PNaCl+Pepper. We fought that fight in part because Google was fine with introducing a whole new platform API in the form of Pepper, and Mozilla instead wanted to minimize the additional complexity by reusing existing Web platform APIs.
> Why does the "open Web" need to stay relevant?
I want a platform that isn't controlled by any single gatekeeper, through which app/content developers can reach all users and all users can reach the apps and content they want. (A sort of corollary is that the platform should be implementable in free software.) The Web is by far the closest thing we have to that.
One problem with a niche browser is that when you have a browser with very low market share, Web developers don't test against it, and with the state of software development technology today, stuff that's not tested in a platform implementation tends to not work on that platform implementation.
Another problem with a niche browser is that by definition it impacts a small number of users, and therefore only a relatively small investment can be justified.
Fortunately there is nothing stopping you or some like-minded group from doing your experiment. Tor has already done something similar and that's gone quite well.