I disagree with this. Yes, we have Chrome, Safari, and Firefox. And a few spin-offs of those. However, functionally, they are all pretty much the same. They all offer largely the same set of features, have substantially identical UX, and in general try to be everything for everyone.
What's missing is a browser for those of use who aren't "everyone." There is an (admittedly small) market for a browser for the privacy-conscious, the ad- and DRM-averse, the hackers, the power users, and those who are delighted rather than scared to see every possibly tweakable knob and button in their settings menu. Let me change the theme, let me rearrange the UI to my workflow, let me block any damn element/site I choose at the network level. Give me an API to extend and modify the _whole_ browser however I want without needing your corporate permission.
What's missing is the "Linux" of browsers.
(To be clear, I am not suggesting that Ladybird or Servo will fill this gap. It's too early to say one way or the other.)