You haven't considered optimization -- with closed modules, the compiler can statically know, for instance, that "Math.sin" is actually the sine function and ca

You haven't considered optimization -- with closed modules, the compiler can statically know, for instance, that "Math.sin" is actually the sine function and can compile that straight down to the hardware instruction. No speculation or guards are necessary. Additionally, your static analysis must be unsound if modules are mutable. The problem is not figuring out which module is being imported, it's figuring out whether the bindings are mutated. CommonJS cannot guarantee that modules haven't been mutated. And yes, you can rename imports. Why is "var { foo: bar } = require('baz')" so much better than "import { foo: bar } from baz;"? They look the same to me, except that the ES6 one is better for static analysis and better for optimization.