[joe-user]

> You can write portable software… and then compile it for each platform to get the best performance and functionality.

Perhaps not what you intended, but I believe this is that exactly what Cosmopolitan is doing: - They are writing portable software in that it can run on various architectures and operating systems - They compile it for each platform, even if the output is a single binary - For the best performance and functionality, they mention several examples of how Cosmopolitan outperforms the native alternatives

I'm sure there are plenty of rebuttals ("Emacs on Windows is a port", "Cygwin isn't portable", "they could make the native version of $WHATEVER more performant", "Cosmopolitan isn't always faster", etc.), all of which are well and good, but would be missing the bigger picture, which is that there are reasons for both approaches (among others!) to exist, an idea that far transcends software.