I don't know enough about the POSIX APIs to comment with any authority on that, but I certainly wouldn't be surprised if a long running software project that has been required to stay largely backwards compatible as it evolved has become crufty.

However, the importance of POSIX isn't really to do with specific design details, but rather because it aids in software portability. This is still a desirable trait. In other words, the answer is to design a better portability solution (N.B. For anyone thinking of posting it, the XKCD standards link is not relevant here, you'd want to design something that had support from key stakeholders before it was implemented, there's no requirement to fragment the market).