Obviously open source is a significant virtue. Mathematica has some strength in closed source, I doubt it could be as self consistent and backwards compatible if people could depend on the implementation as the use contract as opposed to the documentation. It would obviously have had more features earlier, but would that have made the perpetual support, cointegration, or documentation suffer? I'm not blind to the downsides, but I do perceive upsides, and those may contribute to Mathematica's niche.