What's the advantage of writing an interpreter? Go already has an excellent runtime (scheduler, GC, etc)--why should this project reimplement it?

The interpreter could still use that stuff.

One advantage of an interpreter in general is that one important use case for Python is interactive scripting, as data scientists do.

Fair point. I would think it shouldn't require too much work to build a REPL on top of this. Rather than transpiling, you would parse the Python AST into the same runtime Objects that Grumpy constructs statically. Seems straightforward conceptually, though I'm sure it would be complex in practice.