There's an argument that while rendering engines are huge, monolithic undertakings, the actual browsers wrapped around them are fairly trivial. Assuming they use the same rendering engine under the hood, a relatively lightweight, tightly integrated platform-native browser (i.e. Safari or Gnome Web) might be a better user experience than a cross-platform behemoth like Chrome.

Extension support and depth of extension library have traditionally been the big pain point with switching to a minority browser. Moving extension support into a browser independent standard removes a lot of the selective pressure toward a monoculture.