Or accelerated, since development would have ceased to be restricted to Wolfram's employees
Figuratively half of the comments under this post are "I guess it's cute but I can't see anything in there that I couldn't do with Python".
If the open source was so great what Wolfram did would be irrelevant.
For all of Wolfram hubris, Mathematica is just the third kind on the block, after Matlab and Maple. With his ambitions he was obviously aiming for something more culturally relevant.