Just look, for instance, at FPGAs: almost all the tooling is proprietary, very expensive, and very buggy too. Or look at PCB design: Altium seems to be the standard here still, despite Kicad having made huge advances and by most accounts being as good or even better. It took decades (Kicad started in 1992) for the FOSS alternatives here to really catch on much, and only really because PCBs became cheap enough for hobbyists to design and construct their own (mainly because of Chinese PCB companies), and because CERN contributed some resources.
I'm not sure what the deal is with engineers hating collaboratively-developed and freely-available software, but it's a real thing in my experience. It's like someone told them that FOSS is "socialism" and they just reflexively dismiss or hate it.
Most of the incredibly well used robust open source packages are sponsored by large tech companies. The embedded space just hasn't had that kind of sponsorship.
FreeCAD's UI is "a bit crap". The "workbench" metaphor is fine as a metaphor, but the specific way the workbenches are put together in FreeCAD is oriented more towards the way the functionality is implemented than what the user wants to do with it. You have no choice but to understand technical implementation details.
That's just one example, but that attitude to UI in FreeCAD is absolutely pervasive.
I think that's a general problem in the space. The user interfaces in general aren't designed, they're an outgrowth of the direct implementation of the underlying functionality.
> a) have a working mental model of software functioning, so they reason about interfaces differently
You're 100% correct, they do reason about interfaces differently, and thus have wants and needs which are different from those of non-technical users. Those wants and needs are not met by mainstream software, but are met in some OSS software, so it should come as no surprise that such types want to defend that software against the incursion of those who want to make it just like the mainstream offerings!
Like you I have a slightly different perspective here, having been a hobbyist software developer for some years, but having worked in print and design as a day job.
But I still bemoan such things as the awful keyboard handling of the current GTK file dialog, when compared with the old GTK1.2 file dialog. The old one was truly hideous to look at, but handled filtering files by keystroke far better than any current file dialog.
Also 2004: HOW DO I RECOVER LAYERS FROM JPEG?
2024: STOP TELLING ME YOU ONLY SAVE TO XCF AND I NEED TO EXPORT TO JPEG!
Also 2024: crickets
I'm not kidding you. That is exactly what I've been witnessing all these years. Project loss complaints went from daily routine to almost zero. But there's a price to pay.
That approach warns less knowledgeable or hurried users when it looks like they're aiming the gun at their foot, but it still lets expert users follow through unimpeded. Figuring out how to communicate the risk to the user while not removing functionality is proper interface design. Simply not letting people do it is "dumbing it down," and one of many examples of why Gimp is not an appropriate choice for high-volume professional users despite its on-paper feature list.