High security development practices will, as you wisely say, go a very long way. That is not quite the same as some kind of zero-vulnerability "secure" system, however.
No system involving humans is, or ever can be, perfectly secure. At best you can make it uneconomic to attack the computerized parts of the system through networks. You've touched on a number of ways to do this, which are known and understood in the commercial world as both feasible and expensive. This is why Common Criteria has a spectrum of evaluation levels... and even EAL7 does not offer any kind of guarantee of zero vulnerabilities.
Meanwhile, adversaries can and will investigate if their goals can be achieved through human attacks or disruption. There's no need to devote extensive resources to breaking into a system if a carefully placed bomb can produce the same goal, after all.
As you say, this task is by no means impossible. How to go about it is well understood. It's merely very expensive.
Quibbling about how "perfectly secure" is impossible is a complete red herring. Yeah, using known techniques cryptography is not perfectly secure, it will just take billions of times longer than the age of universe and billions of times more energy than exists in the entire universe to break it. It is a distinction without meaning.
Making the computerized parts of a system go from 1 M$ to defeat to 10 G$ to defeat while employing human-error resistant design makes attacking the globally-addressable endpoint go from a cost of doing business to grossly uneconomical. It makes the cheapest, easiest, and most accessible way to attack into a nearly impossible wall and they have to look elsewhere to much less scalable vectors. You do not get the necessary 1,000,000% increase in security from 10% or even 100% improvements here or there. And you most certainly do not get it by listening to the people who are not even within a factor of 1,000x of knowing how to do it right.
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High security development practices will, as you wisely say, go a very long way. That is not quite the same as some kind of zero-vulnerability "secure" system, however.
No system involving humans is, or ever can be, perfectly secure. At best you can make it uneconomic to attack the computerized parts of the system through networks. You've touched on a number of ways to do this, which are known and understood in the commercial world as both feasible and expensive. This is why Common Criteria has a spectrum of evaluation levels... and even EAL7 does not offer any kind of guarantee of zero vulnerabilities.
Meanwhile, adversaries can and will investigate if their goals can be achieved through human attacks or disruption. There's no need to devote extensive resources to breaking into a system if a carefully placed bomb can produce the same goal, after all.
As you say, this task is by no means impossible. How to go about it is well understood. It's merely very expensive.
Quibbling about how "perfectly secure" is impossible is a complete red herring. Yeah, using known techniques cryptography is not perfectly secure, it will just take billions of times longer than the age of universe and billions of times more energy than exists in the entire universe to break it. It is a distinction without meaning.
Making the computerized parts of a system go from 1 M$ to defeat to 10 G$ to defeat while employing human-error resistant design makes attacking the globally-addressable endpoint go from a cost of doing business to grossly uneconomical. It makes the cheapest, easiest, and most accessible way to attack into a nearly impossible wall and they have to look elsewhere to much less scalable vectors. You do not get the necessary 1,000,000% increase in security from 10% or even 100% improvements here or there. And you most certainly do not get it by listening to the people who are not even within a factor of 1,000x of knowing how to do it right.