Python does not replace the Matlab language itself. For as long as we have Matlab code, we will need Octave.
My hope is to make Octave as obsolete as Matlab itself, but so far neither is. Matlab is huge and Python has not unthroned it. It will take a combined effort to do so. Octave is but another contributor to this effort.
In that sense Linux doesn't replace other Operating Systems. But of course it does and that's the sense in which I used the word "replace". As with Windows and Linux, Python is used in lieu of MatLab frequently for new applications as the total number of applications grows.
Programming languages are not quite like operating systems (and even Linux has Wine). When you have a large codebase written in some language, you can't just easily throw it out and rewrite the whole thing in another language. Programming languages never really die out: COBOL is still used these days.
There is a lot of Matlab code out there, and it needs something to run in other than Matlab. That's why Octave is still important.
I didn't suggest Octave wasn't important as an alternative for MatLab. I stated my opinion that Python appears to be replacing MatLab in the niches where it dominates. If there is a new project, Python is likely to be a competitive alternative to MatLab or Octave.
My hope is to make Octave as obsolete as Matlab itself, but so far neither is. Matlab is huge and Python has not unthroned it. It will take a combined effort to do so. Octave is but another contributor to this effort.