Looks fairly cumbersome to extend, and I'm not sure how supportive the interpreter is compared to, say, a typical Python interpreter, let alone a Lisp REPL or Smalltalk workspace.
Am I just whining? I hope not. Monad is a great big leap beyond the DOS prompt, I'll grant you that. But hopefully they are learning something from the industry folks who've been down this path already.
For example, I hope they're spending a week to bring in an expert to sit down at a Symbolics machine for an in-depth tour of the Genera command processor; likewise for a tour of a Smalltalk workspace and tools. Hey, one week for both: 2 days with each technology and then 1 day to solicit feedback on Monad.
Would that be worth the effort? Certainly. They will be affecting developers and administrators for a good long while.