Unix and VMS/NT, the two most popular kernel lineages, were both designed when computers were either isolated or connected to an Internet that was effectively an academic/government walled garden. They absolutely were not designed to deal with the present information war zone where everything is trying to spy/hack/ransomware you and every piece of code is guilty until proven innocent.
Since the Internet went mainstream we've been constantly stuffing wads of chewing gum into their many cracks, adding hack after hack to try to secure that which is not secure. Address layout randomization, pointer authentication hacks, stack canaries, clunky system call whitelisting solutions, trying to shoehorn BPF into a system call filtering role, leaky containers and sandboxes, and so on.
Code signing is an admission that none of those measures have worked.
A secure OS would be built from the ground up with security as a primary concern. It would be written in a safe language like Rust or perhaps even in a system that permits strong proofs of correctness. Every process would run with minimal required permissions. Everything everywhere would be authenticated. The user would have visibility into all this and would be able (if they desired) to control it, or they could rely on sets of sane defaults.
There'd be no need for code signing on such an OS. You could safely run anything and know it would not be able to access things you didn't grant it permission to access. The web JavaScript sandbox is the closest thing we have to that but it's extremely limited. By providing a Turing-complete sandbox that can be generally trusted to run code from anywhere, it does show that such a thing is possible.
(Mobile OSes look like they've kind of done this, but they haven't. They've just stuffed more chewing gum into the cracks in Unix and put a UI on top that hides it. They also "solve" the problem by nerfing everything.)