A system call costs about six times what a vDSO call costs, and nothing in your source distinguishes them. The problem was never the hidden cost. It’s that somebody measured it once and never wrote the number down.
Loop invariants are hard. Keeping a specification in one language and the program in another, in sync, for years, is what actually keeps verification out of ordinary code.
Firefox, Atom, and ActiveX all kept working right up to the point where fixing them meant breaking everything built on top. Eclipse and VS Code stayed fixable by refusing more.