Yeah! As a hobbyI much around with scheme (mostly), Smalltalk (a couple of times a year) and factor (maintaining some old tools I wrote in it).

The result is that if I whine, I whine about the choice of language, not about how a particular part of a language sucks. Extensibility like that should be built in, and then probably mostly discouraged. If you really need to scratch an itch, you will be able to do so without resorting to what usually ends up as ugly, non-composable hacks.