As he mentions, there are lots of projects that counter his claims. Linux kernel. Every Open Source high-level language ever. Red hat.
Wolfram isn't open source because they don't want it to be open source. And you know what? They're allowed to do that! They don't need to justify that decision, it's their decision to make.
Without resorting to "they love money too much", I'll give them another, slightly more honest reason: It's certainly not impossible, but open sourcing an established project is hard. Managing a community of contributors AND a community of users is hard. Keeping a steady stream of revenue while you make the transition is haaaaarrrd. Maintaining all the values that he's mentioned while still keeping all your contributors and community happy: HARD.
And if the cost:value ratio is wrong, you don't do it. And if you have lots of other hard problems to solve, adding one more hard problem with fractional value is silly.
Yes, we would love it to be OSS. Maybe this was posted as a way of never having to answer the question again. Just sad that it isn't a little more honest an answer.