The fundamental problem is that interprocedural optimizations break that model. Optimizations that inline e.g. jQuery code into callers are very important. You

The fundamental problem is that interprocedural optimizations break that model. Optimizations that inline e.g. jQuery code into callers are very important. You end up needing to do optimization on jQuery either way. Additionally, the code that JavaScript JITs generate is not position independent, and it contains no linkage information (symbol tables, etc.). It contains raw addresses of various browser objects within it. You would need to introduce some sort of linker for that to work, which is a lot of complexity.