I'll believe it when I see it, because right now there is no alternative. I wrote a piece of physics software recently and sympy (and indeed python in general) was so lacking in features that I had to write half of the software as mathematica packages, half of it as python. I would have implemented those functions myself but they are actually extremely non-trivial and I wouldn't be surprised if just one of the basic functions I needed had many years of R&D and thousands of lines and edge cases.

Until there is an open source software that can match the raw symbolic power combined with the huge libraries nothing can touch it for a lot of people.