Respectfully, I disagree with that premise. That clearly hasn’t happened with OpenSage, Maxima, or half a dozen other old open source computer algebra systems, many of which were around as long as Mathematica.

If the open sourcing a system works as you describe - why haven’t those system received even a tenth of a polish that Mathematica has, over all these years?

I have my answer: because, even with all its warts, producing something of Mathematica quality requires tremendous coordinated efforts of hundreds of highly qualified professionals who need to be tightly coordinated.

If you try the “next best thing” - a few core enthusiasts plus a large number of users who do occasional contribution you can get to (an impressive) quality level of the above mentioned alternatives, but not higher.