Choice also has costs.

I like the OS analogy, and it highlights the potential benefits well, but we also need to consider the downside.

It would load a huge amount of complexity on web developers. Rather than 5 browsers with a couple of versions of each, you'd need to start testing against a vast matrix of renderers, CSS engines, JS engines, chrome (as in browser chrome, not Chrome) plugins, etc, with versions of each and every one.

Testing software on Linux is hard enough that the economics mean an overwhelming majority of software manufacturers don't bother, or they do but they only support a very limited range of versions, or they release unsupported software that it's up to you to get working. That's definitely not something I believe we want to do for the web.