As I understand it:

MIT code can be freely taken into proprietary fork, with software modified and sold and no patches contributed.

Not sure how often it happens? Has it actually happened and is it a plausible attack? Nowadays at least AGPL is needed to defang "and we will sell it as service rather than selling software" workaround for GPL.

Apple derived OSes, the Windows TCP/IP stack before Windows Vista, PlayStation OSes, Nintendo and Playstation SDK, the BSD stuff taken into commercial UNIXes, just from top of my head, there are surely other examples.

As a practical matter, it means you cant just copy the files into your sources to some subdir, or you cant just link it statically. So you cant release a single-exe version of your project.

These are things some projects like to do, and GPL blocks that. So all thing considered, it may be easiest to just not use GPL code.

you can't copy MIT-licensed files without crediting it either.

And you can statically link GPL code, assuming you release the source to all app users. You might be confusing it with LGPL, which says you can link to proprietary apps dynamically, but have to release source in all other cases.

You can also link to proprietary apps statically if you provide the object files: https://softwareengineering.stackexchange.com/questions/3127...

You can use GPL code however you want.

You have only obligations when you distribute the results. Whatever you do in your private basement is completely unaffected by that. The GPL gives you freedom to do whatever you please. That's the whole point of it!

When you distribute binaries resulting form work based on GPL code than the people receiving those binaries (and only them) are eligible to get alongside the source code which allows them to rebuild the binaries you distributed to them. That makes sure that those other people also have the same freedom as you to do whatever they please with the programs and code. So the GPL preserves freedom among users of GPL code.

You're thinking on the LGPL.

The GPL is the prototypical copyleft licence, designed to encourage the liberation of software, in part via its dependencies.

Apple's OS is a fork of BSD without the fork being open source.

It’s more complicated than that. It was not a fork, they just copied some parts of FreeBSD and OpenBSD. The kernel is XNU, and descended from Mach. Also, Darwin (the base OS, which includes the BSD bits) is open source. What isn’t is the GUI on top of it, which comes from OPENSTEP.

The open source part of Darwin/XNU doesn't deserve to be called a "base OS" since version 6, when Apple closed the sources of a lot of drivers. There's no hardware that will boot these systems if you restrict to what is available under the APSL.

https://web.archive.org/web/20091118172944/http://www.pureda...

GPL software, and extensions to it, are free and will stay like that.

That can't be said about software under the above mentioned licenses.

I'm not sure how the GPL guarantees that it will stay like that anymore than those other licenses?

You can always relicense GPL content as long as everyone agrees. You can also change it to source on request versus always accessible.

If you wrote the code, you can refuse to relicense.

key is "as long as everyone agrees"

OP also said it was a sign of 'quality'. Not sure how GPL promises that over other OS licenses.

I'm the OP and I'm not a native speaker but to my knowledge "quality" denotes in English besides a few other things the meaning of "value" (in the sense of worthiness) or (related to that meaning) "excellence". Something being high-quality makes this thing valuable.

I didn't mean "code quality", or the like, as a license can not enforce that obviously.