I can't speak to your subjective question but there's a lot of software without automated testing that people are completely capable of modifying without breaking. Ask me a real question and not one about the confidence YOU get from something and I'll see if i can't answer it.

I believe there is a rather apt quote by someone whose name escapes me right now (C.J.Hoare maybe?). It goes (paraphrasing): "There are two ways to write software. One is to write code that contains no obvious deficiencies, and the second is to write code that obviously contains no deficiencies. The second way is much harder"

I always took this as an argument for smaller, composable components that value simplicity over complexity, dumb over smart and composable over monolithic. I personally (not quite there yet!) try to write code in this manner. I find that the more straight forward and simple the code, the less need there is for tests because I can hold the whole of the code base in my head.