Why say Win32? Why not say WebKit and jQuery are the new Linux? It's actually a better analogy.
Software monoculture is not inherently a bad thing; it can be a good thing. Re-inventing the wheel isn't progress. The world is much better off with Linux than 20 different Unix variants. And the world is likely better off with WebKit than dozens of different browser variants.
The issue isn't having a software monoculture, it's who is in control of it. The situation with both Linux and WebKit is that we have all the benefits of a monoculture and very few of the downsides.
Re-inventing the wheel isn't progress, but coming up with radically better solutions is. And that's much harder to do if you have to not just match a specification but also be bug-for-bug compatible with an existing large codebase that you have to reverse-engineer to achieve said compatibility.
Because that would be a terrible analogy. Using linux as an example actually goes against your point. We do have "20 different unix variants". Just because you live in a linux bubble, doesn't mean there isn't a world outside of that bubble. Almost all software that runs on linux isn't using the linux api, they are using a gradually munged up collection of APIs from over the years, including posix, bsd and sysv APIs. That software works on on dozens of different systems. Software written to a common API that is implemented by dozens of different systems seems a lot more like an argument in favour of more rendering engines implementing a common API.
Software monoculture is not inherently a bad thing; it can be a good thing. Re-inventing the wheel isn't progress. The world is much better off with Linux than 20 different Unix variants. And the world is likely better off with WebKit than dozens of different browser variants.
The issue isn't having a software monoculture, it's who is in control of it. The situation with both Linux and WebKit is that we have all the benefits of a monoculture and very few of the downsides.