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No, that's not the goal of open source,That is the goal of FSF opensource projects - protect the rights of the developers and users to review and modify code.
> The end consumers got better software as a result.
I disagree. If they don't have access to the accompanying source code, it is an inferior product.
> I don't see how the GPL helped anyone in the case of Apple and bash.
Thanks to FSF license, any user can ask the distributor to provide the source code of an FSF licensed app. In our example, that's Apple and they cannot refuse to give you the source code of bash along with the modifications they have made. Which brings me back to the point that needs to be emphasised - GPL and other FSF license was not designed to help businesses and corporates. It's aim is to ensure the freedom of developers and user to review and modify code is preserved.
> If I find a bug in my Apple-supplied copy of zsh, am I prevented from fixing it like RMS and the printer driver? No.
Yes, if Apple refuses to give you the source code of their customised zsh, then you are indeed being prevented from fixing any bug or adding any new feature! I hope you finally understand - FSF license ensures that Apple (or anyone else) cannot refuse you the source code. Other opensource license don't come with such guarantees.