Sure, Mathematica has had a long head start. I don't see the gap widening at all, though. For me, e.g. SymPy (and more specialized algebra systems for e.g. quantum mechanics I've built on top of it) now match and sometimes surpass what I could do with Mathematica. More importantly though, Mathematica's closed nature makes it quite hard to integrate with other systems and workflows (as others have pointed out in this thread). The benefits of the open scientific ecosystem around Jupyter far outweighs the few areas were Mathematica still leads in functionality. This is a subjective assessment, of course.