There's a whole lot more to Unix and open source than GNU. macOS has current versions of many open source tools, such as zsh, awk, sqlite, tar, git, and xz.
If one is being pedantic, GNU is not even "open source" but rather is "free software," and macOS certainly is not free software.
On Linux, sure, but not on other Unix OSes (like *BSD)
I'm not saying it's an easy problem to solve. Just that Apple aren't dropping support out of respect to their customers. They're doing it because it's cheaper.
Apple seems to be really good at maintaining their profit margin.
Here, they not only realized it had become popular, but GNU licensing gave them an excuse to switch the default without much thrashing about.
The fact that I can `chsh -s /usr/local/bin/zsh` and macOS doesn't freak out is quite nice.
Nowadays macOS needs something like Homebrew to fill the gaps. It's still slightly better than Windows + WSL, but Microsoft is catching up, while Apple seems to think that iPadOS is the future.