I think the big turnoff for me is its closed-source nature
Well, the issue of open-vs-closed is less clear with things like MATLAB. Sure you can't download it for free-as-in-beer, but if you own a copy, you are free to inspect its source code, as MATLAB is mainly written in MATLAB (same for Mathematica and other similar packages).
A subset of non-performance-critical functions, and things like argument validation layers, are written in m code that you can read. Most of the actual work happens in obfuscated p code or calls into compiled libraries. Many of those libraries are available independently of Matlab, but you can't see how they're calling them, or important details like how the Matlab interpreter or JIT compiler work.
Well, the issue of open-vs-closed is less clear with things like MATLAB. Sure you can't download it for free-as-in-beer, but if you own a copy, you are free to inspect its source code, as MATLAB is mainly written in MATLAB (same for Mathematica and other similar packages).