It’s not even that these evangelizers think we should all be using the same browser. It’s that there are currently only two realistic choices: Chrome (and it’s derivatives) and Firefox (and it’s derivatives). There is safari too, of course, but it hardly compares to either in it’s current state.
Given those two choices, only one of them is in support of the open web. The other is literally trying to add DRM to the web.
As to your first point: I agree that here it may be preaching to the choir and that we all get it. But it has such a small marketshare, I’m not sure it is good for those encouraging it to be quitened.