Fair enough, it's true that they didn't start from scratch, but iOS today has evolved far beyond those original borrowed elements and is now much more than just

Fair enough, it's true that they didn't start from scratch, but iOS today has evolved far beyond those original borrowed elements and is now much more than just a kernel and some early shared user space code. Even if that were not the case, it is still surprising that a company with Apple's resources would continue to favour writing new parts of iOS the same way 14 years after its first release.