HeeksCAD / CAM is also built on OpenCascade, but is really more of a hobbyist app ATM… its kinda being patched together haphazardly without any real thought to workflow or proper structure. Thus, despite the active development, it will take a loooong time for it to become usable for real-world pipelines, and even longer for it to get adopted to any “critical mass” sort of level. But its pretty cool for what it is right now!
The difficulty with marrying OSS with CAD is the business model. Sure, there are a lot of small businesses, freelancers, educators and entrepreneurs out there who have a need for this kind of tool, but most have it within their budget to purchase such a tool with a reasonable ROI. This is because you usually use CAD to design something to sell, either the design itself or the product which gets designed, and thus have a profit potential. You typically don’t just put a set of blueprints together for the artistic expression of it (though I suppose some do…) Also, there are a few very good inexpensive options available to fill those market niches (with Alibre starting at ~$100 I think? And KOMPAS pretty cheap too - along with several free 2D options which often suffice for small operations.)
The reason for the level of success with Blender is that its a tool that is focused on producing artistic expression, which can be used either for profit or for pleasure. Thus it has a large amateur userbase as well as many who use it in their business workflow. But its the non-proffesional dilettante who really got it rolling at the start. Then the value-add folks come alongside to keep it going. Could the same thing apply to a CAD app? Not sure, but doesn’t seem likely.
EDIT: The other thing that goes alongside the economic aspect of CAD is the need for specific features / functionality. While the feature set of CAD is somewhat more established than DCC, the need for given features would be much more urgent being driven by customer requirements. Would an OSS project be able to deliver when such need arises?
I know that’s one of the frustrations for many with Blender: features either missing or partially implemented, not allowing them to be used to meet a customer request. (I’ve got one right now where I could use Freestyle to do something, but its not ready for prime-time just yet, so I’m likely having to forego the sale.) With a CAD app, the stakes seem like they’d be higher, so one would just go get a commercial app to get the job done, asap.