Yes, some people won't give back. However, a project like llvm/clang, most people do give back because sharing is cool, and also it makes their job easier because they don't have to repatch for every new version of the compiler. Sony's team developing the Playstation 4 wished at times they could use newer versions of llvm/clang for their software, but couldn't because their version had so many patches [1]. Because of this, after the PS4 was released, Sony developers sent a bunch of patches upstream, and work pretty closely with the rest of the developers. Some bits and pieces may not get merged upstream, but that's the same for any project, GPL or not.

[1] http://llvm.org/devmtg/2013-11/slides/Robinson-PS4Toolchain....

On the flip side, code from permissively-licensed projects (like LLVM/clang) can be incorporated into free software with GPL-incompatible licenses. I personally consider that gain to be more than worth the risk of proprietary forks.

Also, the licenses of neither GCC nor LLVM apply to programs compiled by either project. I don't think that's what you meant by the copyleft sentiment, but the wording of that bit is weird and it's worth clarifying just in case.

Please note that no company has the right to strip your copyright or license away. But they may chose to extend your software without giving you access to the extension, if that is permitted by the license.

"after having switched to Musl" - Musl is not GPL either.

The BSDs have been used in proprietary software, but the open source projects continue as the centre of development.