I am not focusing on bug bounties, I am focusing on cost of discovery which seems like a pretty good metric. Bug bounties just provide a means of learning the cost of discovery that is publicly available and where over-stating is harmful. If you have some other good quantitative measure of security that is publicly available and where over-stating is harmful that would be very helpful.
I stated in a parenthetical that I did not believe they would actually root every device in practice. I used numbers, you can change the numbers to whatever you believe. If someone wanted to mass infect devices using a zero-click kernel code execution, how many do you think they would be able to infect? Let us call that X. $1M is order of magnitude the cost of discovery (since bug bounty ~= cost of discovery) for such a compromise on iOS. Divide $1M / X, that is the per-unit value. Does that number seem good? I said $50 is probably adequate. Therefore, for that to be adequate given this model, you would need to expect a zero-click kernel code execution deployed to mass infect would to infect 20,000 or fewer phones. Do you believe this is the case? If so, then your logic is sound and in your mind iOS security is adequate. It is not for me, since I do believe it would only infect 20,000. As a secondary point, that is only the cost of the compromise with no infrastructure. If they spent another $1M developing techniques for effective usage of the compromise such as better ways to deploy, infect, control, hide, other compromises, etc. how many do you think they would be able to infect? Let us call that Y, Y >= X. In that case I would do $2M / Y to determine the adequacy.
As a counter-example, large-scale ransomware attacks which extract minimal value from large numbers of people occur and have been increasing in frequency and extracted value. Why aren't there more given how easy it is? I don't know. Why didn't somebody crash planes into buildings before 9/11 or drive trucks into people before the 2016 Nice truck attack? These attacks were not very hard, possible for decades, and seem to be highly effective tools of terror, but for some reason they were not done. Hell, it is not like anybody can stop someone from driving a truck into people right now, why aren't terrorists doing it every day given we saw how effective it is and how hard it is to prevent? My best guess is that so few people actually want to engage in terrorism or economic hacks that only an very tiny fraction are done at this time.
This leads into the next point which is that state actors are not the only entities that CAN break into phones; financing a $1M cost of discovery is chump change for any moderately sized business. The government is just one of the few entities who want to as a matter of course and face minimal repercussions for doing so. If you are not the government and hack people for financial gain you can go to jail; not a very enticing prospect for most people. This means that the impact of a compromise is not different, it is just less probable at this time. However, that is not a very comforting prospect since it means you are easy prey, just nobody is trying to eat you yet. And this ignores the fact that since any particular target is easy, if someone is targeted in particular they are doomed. Essentially, not being compromised is at the mercy of nobody looking at you funny because if someone wants to compromise you they can. To provide an example of why this is a problem, if I were a terrorist organization, I would be running an electric power generator and transformer hacking team with an emphasis on bypassing the safety governors and permanently destroying them. It does not matter that there are more economic targets to hit, as long as they choose one in particular they can cause incomprehensibly large problems.
As for Google's security, if I use my default security assumption (based on experiences with other security organizations) that a skilled red team with $1M would be able to compromise and establish a persistent presence with material privileges and remain undetected for a week, then I believe their security is inadequate since I believe such an outcome would easily be able to extract $1M, let alone damage if the goal were just destruction. If the goal were pure damage, I believe that such presence, properly used, should be able to cause at least $100M in damage and I would not find it unreasonable if it could cause $10B in damage if the goal was optimized damage to Google in both material and market ways with no thought for the consequences if caught.
To separate this out for you, there are two primary statements here:
1. The damage that a skilled red team can cause assuming it has a persistent presence with material privileges and remains undetected for a week.
2. The cost of creating a persistent presence with material privileges that remains undetected for a week.
I assert (1) is probably ~$100M. I assert (2) is my default of $1M. Any situation where (1) is materially higher than (2) is inadequate in my opinion, so a convincing counter argument on your side would be convincing me of a value for (1) and (2) where (2) is higher than (1). I find it unlikely you would convince me of a lower number for (1). So, you would need to claim that (2) is ~100M for Google. If you believe so, what is your justification? The minimal standard that would cause me to consider further (not convince, just not directly dismiss), which you are under no obligation to provide, would be: You stating that you talked to an internal security person at Google and they firmly claim that (2) is higher than 100M (I will take you at your word). If you do not know what to ask, you can try: "If you were in charge, would you feel comfortable going to DEFCON and Black Hat and putting out a prize for $100M if anybody could do (1)?". The other requirement is you stating (again, I will take you at your word) that you talked to an internal security person at Google and they inform you that this has been tested internally using a red team with resources in the general vicinity of $100M or more. There are potentially other strategies that might pass the minimal bar, but that is one that I could think of that would be pretty solid. Again, I am not demanding you do so, but if you wish to engage on this point then I don't think any other type of response is particularly productive.