For the greater good of the world he shouldn't OSS it, but make it free! I'm mostly a R/Python statistics type, but a multi-day course in Mathematica left me in awe. It is great software. The way you could make models dynamic in a user interface kind of way was way ahead of it's time. It's pricing was prohibitive though and we chose not to pursue it.
I think Mathematica is quite fantastic (took me some time to understand it's patterns). I don't use it currently because 1) A lot of my current work is numerical and not symbolic -- for which Mathematica isn't great; so I use Python/Julia 2) From a long term perspective, I wouldn't want to invest a large fraction of my personal computing efforts into a proprietary single point of failure.
TBH, with regards to paying for software, my psychological motivation is still a work in progress (so the comments above are directed at myself as much as at others), but I'm trying to build a habit of doing it more -- especially to support free software. I care about computing too much -- I would hate to look back on this twenty years later and rue the loss of computing freedom because we were too stingy to invest a hundred bucks a year in the appropriate places.