Well, something like "privileges" seems like a design that is a holdover from a time of shared computing. We've adpated it for other uses, and for some people it's become some sort of security religion. If we assume it exists for the purpose of protecting users of the same computer from each other, it has no need to exist in most cases. Computers are affordable enough that we can all have our own. There is rarely necessity to share computing time.
Plan 9 settled the issue.
But sometimes I think some people have come to love the mental challenge of UNIX permissions. It is like a game to them. I made a function that produces all possible permissions for a file, including the nonsensical ones, so I could display permissions as octal instead of the usual ls format. Have you ever thought about how many possibilities there are? Too many.
The setuid concept is interesting and must have seemed quite innovative at the time. Doesn't surprise me they filed a patent for it. But today, I could just as well do without the complexity.
As someone who supports ordinary programmers, expecting smarts is not the right track. expecting average and mistakes is the right track. If your foundation is not robust by default, your systems will not be robust by default either.
To mix metaphors: Better to design foundations that keep the gloves on by default, then when you need to, take them off. You can see that design philosophy in Mercurial.
Suppose - for argument's sake - that C had two different pointer types: checked pointers for arrays and unchecked pointers for arbitrary memory and that conversions between the two were relatively seamless. What difference would that make, 97% of the time?