[bmitc]

(1) is true, but Mathematica is also supported because it is paid. (2), (3), and (4) are very iffy stances. Open source projects also fail when leaders move on, and it's actually less likely for a company. (5) has nothing to do with open source, and Mathematica is extensible.

I ask because I always see open source thrown around as if it's some paragon of quality and productiveness. In reality, the actual usefulness of a product is fairly independent of its open source status. And rarely does it matter all that much to a project that a software component is open source or not.