The short answer is that changing the type system to allow multiple inheritance (traits) involves solving some really hard and open-ended problems (eg method sp

The short answer is that changing the type system to allow multiple inheritance (traits) involves solving some really hard and open-ended problems (eg method specificity). Solving this is probably breaking, and requires a ton of work on a problem that might not even be solvable. If I had to guess it will be solved by the language that replaces Julia in 20 years.