1. The XNU kernel isn't novel, and isn't used by anyone other than Apple. Open-sourcing the NT kernel would effectively kill the project, since everyone would port the Win32 bindings to Linux (same thing would happen if Apple open-sourced their userland software)
2. Apple didn't get to choose whether they could make LLVM/Clang open source. They were forced to release it under GPL, and if you actually look at the license for those projects you'll see that Apple's contributions are dual-licensed.
3. What chance would FoundationDB have if it wasn't OSS? Why would Apple maintain a stale, internal fork of Apache Cassandra?
Nether llvm nor clang are GPL, dual licensed or otherwise. They're Apache 2.0[1] (or formerly a quasi-MIT/X11 license[2]), as is swift[3].
The binaries of the compilers they release with the platform are also built with internal forks that have some differences with the public llvm/clang/swift trees and that wouldn't be possible with gpl.
1. NT kernel is neither novel nor used by anyone other than MS. Open sourcing the NT kernel would effectively kill the project as its security issues would be fully exploited with wide range impacts on literally everyone.
2. LLVM/Clang are not GPL licensed, they are apache 2.0 licensed. Open sourcing contribution is just open source contribution, it is not about whether you get to choose the license or not.
3. Well, SQL Server is a pretty good demonstration that proprietary database software can survive for decades.
* MS still refuses to open source its windows kernel to the public. Apple has its kernel accessible by the entire world.
* MS still refuses to open source its c++ compiler. Apple has swift built by the open source community and there are llvm/clang.
* MS still refuses to open source its sql server, when Apple has FoundationDB for the world and actively contributing to Apache Cassandra.
We are talking about a company that for many many years called open source a cancer.