Suppose that there would be an open source CAD software that does all the basic things right (like Blender). I agree with you that probably nobody would pay big money for that. That's one reason why it doesn't exist today. But suppose that it exists. Why then would all those "plugin makers" invest in a specific proprietary software when they could extend the open source software instead? Why would users want a proprietary software when they could use the open source software instead?

To me it sounds like professional CAD is really hard and nobody can "just make it" in a few years of work. That's the reason why open source CAD doesn't really exist today in the professional space. But once it exists, it will eat the world. The transition phase will still take dozens of years because of all the work invested in existing platforms. We will probably see something similar to the adoption of Linux in the commercial space where it simply makes sense to use Linux instead of developing your own OS or licensing some other OS.