Imagine if I spent the next year writing an immensely complicated optimizing compiler for Arc, and succeeded in making it 2x faster than code generated by the compiler the MzScheme guys have spent so many years writing. A lot of people would be more impressed with Arc then. And yet I'd have pretty much wasted my time. Arc is already acceptably fast. So at the end of this exercise I'd have produced a language that was better in ways users didn't care about, and no better in the ways they do (language features).
Instead I work on language design. Like JMC's code, this kind of thing is easy to copy-- once someone has already shown it to you.
You know, when I write about how one should work on the problems that really matter instead of the ones that are prestigious, people read those essays and think "yes, obviously, that's exactly what I'd do." But it's not till you try working on unfashionable problems that you see how immense the pressure is to work on fashionable ones.
Just as well I've avoided saying most of the "things you can't say," or 90% of the people who read that essay and think "hear, hear" would hate me instead....