It applies to both sides of the argument, with organizations keeping things to themselves or others avoiding codebases for the inverse reason.
Given that this issue applies to all proprietary/non-opensource software, which is the overwhelming majority of software out there, you can hardly blame GPL3 for this fact of life.
They presumably can salve that horrible wound with all the money they get (not make or earn, just get) from their chosen modus operundi.
I don't see all the efforts that go into *BSD, Windows, macOS, etc... as being wasted just because Linux is available, nor would I consider the effort to try something completely new in os/kernel design that starts from a clean slate a waste, even if it doesn't end up becoming wildly popular. Not everyone who has the capacity to work on a duplicate effort would be able to contribute to the original, and the original will never be able to perfectly meet the goals/needs of everyone.
The license angle with Apple did happen because of GCC's switch to GPLv3 but that came years later - and GCC still wasn't (and AFAIK still isn't) designed to be used like LLVM or (later) Clang, like using it as a backend for IDEs, analyzers, language servers, etc. The two projects aren't interchangeable with only the license being the thing that differentiates them.
I don't know how you can eat GCC's lunch. I feel like in this analogy GCC eats from a communal trough and is very happy to share. A company that sells licenses can lose customers, the GNU project just wants the world to benefit. GCC is still ubiquitous and has plenty of developers and mindshare, what more could they want?
I'll note that GPL never says anything about upstreaming. It can certainly make life easier if you want to keep your patches up to date and hand off maintenance responsibility, but it suffices to make a written offer of source code under the same license (on a conventional medium) to your customers to comply with any GPL version. You're never obliged to upstream or even publish your code.
Respectively, LLVM fills it's own gap nicely. It has the classical BSD-like issue where hundreds of unique and disperate forks exist with no roadmap to bring them together, but that's also technically a "feature" of each license and not a bug. If GCC developers felt their job was obsoleted by Lattner, there wouldn't be a reason to update and maintain it. The end result has been healthy competition, accountability for either team, and better optimizations for everyone using either compiler.
[1] https://rust-gcc.github.io/
[2] https://github.com/Rust-GCC/gccrs
[3] https://blog.rust-lang.org/2024/11/07/gccrs-an-alternative-c...