The work of a tenured professor is more than 20% time. A normal teaching load is 4-5 classes a year, plus significant committee work, student advising, etc. It takes an enormous amount of time. And it can be awesome, fun, and many of my colleagues love doing it. But it doesn't result in creating a free open source alternative to Mathematica.

I think people underestimate how much work a professor does. Everybody in my department who started a company either did it before starting at the university or they did it while on sabbatical.

Good luck!

If you do the bare minimum work as a tenured professor you're going to get an awful lot of people extremely mad at you at all levels of the academic hierarchy.

If you do the bare minimum work and you're spending a lot of time on your own company, at my university, you will almost certainly get a pink slip. Doesn't matter if you have tenure. We are not allowed to work more than one day a week on such things, and that requires approval, which probably won't happen if we're not publishing, have terrible teaching evaluations, and aren't doing a full share of service and advising.

This is precisely correct. And even if I could get away with it, I would not feel that it is morally right (for me at least).