I think you put your finger on it... people talking about Mathematica/Wolfram Language as a CAS are really chasing an idea of what it was 10 or 15 years ago.

Maybe I'm naive but I think that we really can just "all get along" -- notebook-based programming is just one example of how ideas can incubate in closed-source projects, and end up benefiting open-source projects. I'm hoping knowledge-based computation ends up being another.

And the reverse happens too -- Light Table is tremendously exciting, and I hope WRI can learn from that as it develops.