>> Really? I don't see IntelliJ Idea suffering much from NOT being Open Source

Yeah definitely. Idea & Visual Studio (among others) are top quality products, and yet both nothing more than a chapter 11 filing away from being vague memories.

Contrast adoption of Idea vs Eclipse.

Consider when Windows is no longer relevant, VS will necessarily degenerate to being a niche product (even though it is arguably one of the finest IDEs available at any price today).