Secure in what terms? Security is always about a threat model and trade-offs. There's no absolute, objective term of "security".
> AWS has a rule that the same person cannot have logical access and physical access to the same storage device.
Any promises they make aren't worth anything unless there's contractually-stipulated damages that AWS should pay in case of breach, those damages actually corresponding to the costs of said breach for the customer, and a history of actually paying out said damages without shenanigans. They've already got a track record of lying on their status pages, so it doesn't bode well.
But I'm actually wondering what this specific rule even tries to defend against? You presumably care about data protection, so logical access is what matters. Physical access seems completely irrelevant no?
> Very few companies have enough IT people to have this rule
Maybe, but that doesn't actually mitigate anything from the company's perspective? The company itself would still be in the same position, aka not enough people to reliably separate responsibilities. Just that instead of those responsibilities being physical, they now happen inside the AWS console.
> The AWS KMS is vastly more secure than what almost all companies are doing.
See first point about security. Secure against what - what's the threat model you're trying to protect against by using KMS?
But I'm not necessarily denying that (at least some) AWS services are very good. Question is, is that "goodness" required for your use-case, is it enough to overcome its associated downsides, and is the overall cost worth it?
A pragmatic approach would be to evaluate every component on its merits and fitness to the problem at hand instead of going all in, one way or another.
