Realistically the security research community who find things like phone exploits are very well accustomed to a lack of source code, and in many particular niches of infosec -- it's a baseline assumption for almost all analysis work. Apple's security team is the reason the iPhone is an extremely secure device, not because it's closed source, and not for lack of trying. Go ask people who find major security vulnerabilities -- binaries are rarely a true obstacle, time and mitigations are. (And you'll probably come away surprised at how easily most systems fall to even basic intrusive analysis, no source code needed.)

Source code is merely convenient for the security community in cases like this. But it's hardly a requirement at all, and you can absolutely build a secure system if you design for it.

All that said, I think source code for a device as personal as a phone is largely more of a question of ethics and this is what Purism heavily leans on as an angle for their marketing. I think that's a good angle, independent of any security aspects: owning the code and having the right to it (even if you don't ever touch it) is an important aspect of owning a device as deeply personal as a phone. People like us who believe that should totally advocate for it -- I think it's a fairly simple case to make. But it really has little to do with _security_ on its nose, if you ask me.

(Also, if your government mandates backdoors, they will not be thwarted so easily by simply recompiling AOSP or whatever and loading a new ROM, if they do not wish to be. Any belief to the contrary is a fairy tale that people like us enjoy telling themselves. But I'll give you a hint: writing a Makefile cannot get you out of jail, and you are not that good at covering your tracks.)

> Realistically the security research community who find things like phone exploits are very well accustomed to a lack of source code

Realisticaly, though, a manufacturer or intelligence agency will be the one paying security people for thorough reviews. Anyone doing this in the open would do this to build a reputation or for their own security, and will need much more time to review something that first needs to be reverse engineered. They will almost certainly not be able to look at everything.

It's not a logical argument to say that we security people are used to reading obfuscated binaries anyway and conclude that therefore no fewer bugs will be found compared to the situation in which it is open source.

This is one of the rare instances in which there actually is a liquid market for viable vulnerabilities and exploits, so no, I don't think it's safe to say that the only people doing this kind of research are in the IC or vendors.

Apple is the gold standard of mobile security and also a shining example of a closed source, proprietary ecosystem. This is coming from a dedicated Android user who has a preorder for a Librem 5. I did not order it for security reasons, I ordered it for ease of development. If I had sensitive information on my phone, it would be an iPhone. I just prefer living in the wild west.