So to be clear, you are arguing that we cannot blackbox anything and see what network connections it's making or amounts of data being sent to whom or timing of it because proprietary? Nor for that matter find security vulnerabilities that lead to jailbreaks and then further deep dives right? Since it's proprietary that means nobody can possibly find any issues? You might want to think about this one just a little bit longer.

Seriously, the fundamental issue with proprietary is maintenance, ie., not finding but fixing (in a good way) problems and then making those fixes available to other users. Adding features to scratch niche itches is another, though arguably not as critical a matter. But for merely reverse engineering, decompiling, probing memory, fuzzing and all that lack of source code is effectively zero barrier. If it wasn't then source/algorithm obscurity really would be effective for security rather then a bad joke.