One could make a number of arguments about at least half of these. Racket/PLT has been free for a long time but ten years ago, a friend of mine simply left it for Common Lisp for the sole reason of speed. Gambit is great - I'd say almost as good as Chez - but there's no proper modules or native multi-threading. The Scheme->C model is also cumbersome; reintroduction of a native compiler was promised but if I'm not mistaken, still hasn't happened (although some other interesting things did - the recent mobile code support is very interesting!). T was a research vehicle but apparently the closest thing to an accessible environment for it was A/UX on Mac. Say what you want about IBM PCs but they won.