Indeed, there are many high-quality alternatives (sometimes described as "MATLAB clones" back in the day) that never gained bigger traction.

Among modern alternatives that don't strictly follow MATLAB syntax, Julia has the biggest mindshare now?

GNU Octave, as a superset of the MATLAB language, was (is) most capable of running existing MATLAB code. While Octave implemented some solvers better than MATLAB, the former just could not replicate a large enough portion of the latter's functionality that many scientists/engineers were unable to fully commit to it. I wonder whether runmat.org would run up against this same problem.

The other killer app of MATLAB is Simulink, which to my knowledge is not replicated in any other open source ecosystem.