Look, Sage is great, I hope Sage does really well and eventually duplicates the entirety of our functionality. But at the current rate it'll take probably hundreds of years. Sometimes, profit-subsidized research is just much, much more productive at driving innovation.
True in a deep sense. In principle one could collect all the open-source components that make up Sage and recreate it independently. Sage resembles gluons more than it resembles protons and neutrons -- it holds together things created by others.
If I were you, I would want to avoid describing Wolfram's present circumstances as a monopoly. The word has certain legal implications one would do well to avoid.
The legal issues turn on how the monopoly was acquired, not its mere existence. In any case, my original point was that it's unwise to describe one's company's position as a monopoly -- it sounds predatory, and it can only attract the attention of corporate lawyers.