Interesting hypothesis, but it doesn't explain the pattern. That's also true of a million other categories of dangerous equipment, and yet almost none of them

Interesting hypothesis, but it doesn't explain the pattern. That's also true of a million other categories of dangerous equipment, and yet almost none of them lock out competitors by using proprietary interfaces. A shoddy saw blade could kill me, too, but I still can go buy any blade from any manufacturer and put it in any table saw, and it'll work. OTOH, there are gratuitous incompatibilities galore as soon as you get into software. Network protocols, file formats, executable formats, and so on. What could explain the situations where we see compatibility versus incompatibility? If it's obvious and visible, and Average Joe can make it fit (like with a hammer), they go ahead and make it compatible. If it's secret and internal and they can hide the differences in ways that Average Joe can't work around (like software), they feel free to make it as incompatible as possible.