As a counterargument, the web platform has multiple "frontends" (multiple, unrelated codebases that interpret HTML and JS and CSS, not just multiple engines/ren

As a counterargument, the web platform has multiple "frontends" (multiple, unrelated codebases that interpret HTML and JS and CSS, not just multiple engines/renderers), and the argument to avoid the monoculture seems good there. Although, hm, I suppose you could argue that if there were one libhtml5 and Gecko and Blink and WebKit all used that, but they had different "backends," that might be even better than the current situation - that the value of having multiple web browsers is in the equivalent to the backends, not in the equivalent to the frontends.