For now

For last 20 years at the least, and the gap widens if anything

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.

Sympy is great (especially the ability to automatically codegen numerical expressions from symbolic ones), but I've found it to be orders of magnitude slower than Mathematica for some relatively simple problems. I also find the sympy documentation to be somewhat sparse and hard to navigate.