My adjacent conspiracy theory is that the NSA and other state agencies do both original research and pay hackers for exploits that Apple hasn’t yet discovered.
Like how the NRO used to design and launch satellites that cost more than aircraft carriers but are now working closely with private companies like Maxar to find more economical solutions.
https://www.maxar.com/press-releases/nro-awards-maxar-a-10-y...
Your question assumes two things: (1) That Apple intentionally leaves vulnerabilities in the stack, and (2) that Tim Apple is occasionally willing to share this candy with governments.
Having worked at Apple, I don't believe (1) can be true. Not only is it extremely unlikely that it could be kept a secret, but Apple's thing is "obsessive control", a mindset borne of organizational PTSD which originated with its near-death experience in the mid-to-late 90s. The Apple I know would not risk intentionally leaving back doors unlocked for enemies to find and leverage.
As for (2), the existence of a "Binder of Vulns" by nation-states would expose Apple to existential risk. It's possible that it could be kept secret within Apple's walls if it were never used, but once shared with a government it could not be contained. The splash damage of such a discovery could easily kill Apple.
Knowledge of a binder of vulnerabilities is perhaps one of the greatest secrets that must be protected. Wikileaks releasing the Vault 7 leak was the death knell of Julian Assange. It proved such a binder exists in great detail.
I don't hate Apple, but assuming they can't be reached, seems naïve.