Sergey Brin was only working on open sourcing the "pure language" part of it [1].
I share the dream of an open source Mathematica. It's so rich, elegant and powerful, using it would be a no-brainer. But in its present state? Nope. I've been bitten too many times by the proprietary nature of Matlab, in which I have invested a lot of work, to repeat the mistake. More specifically and from the top of my mind, I found that proprietary means:
- I cannot run my code on my personal PC
- some people I'm collaborating with cannot use my work
- when I move to a different job, there is a good chance I cannot reuse my code
- even when my new workstation (or my colleague's PC) does have the software installed, there is a good chance the version is incompatible with my code or the license doesn't include some toolbox I'm using
- some work I want to make freely available is of no use to someone who cannot afford the software
Notably absent from the list is the notion that I cannot check what is happening in the proprietary code. That is because the Matlab functions I needed to check were written in Matlab and perfectly readable. I'm sure the issue is real though, and I would be concerned if a new kind of science developed that produced results through opaque processes in closed source proprietary software.
[1] http://www.bestofama.com/Stephen_Wolfram/qisot