I've released software under the GPL, but I never will again. Don't get me wrong, I love open source, but GPL's license terms are unaccountably toxic, creating an island that I am pretty sure that original GPL authors never intended.
So I started by wanting to contemplate a licensing change for a new library I'm working on, to move from the very loose and liberal MIT license, to something with a few characteristics I like -- namely patent protection and a "builtin" contributor agreement. I'm speaking of course of the well-respected and well-regarded Apache License 2.0.
The problem is, I ran into a complete and utter roadblock.
I want my software to be maximally usable by as many folks as possible.
There is a large installed base of software released under the GPLv2. (Often without the automatic upgrade clause.)
Now I'm not a big fan of "viral licenses" in general, but I get that folks want to have a copy-left that prevents folks from including their work in closed source projects. I get it, and it's not an entirely unreasonable position to hold, even if I think it limits adoption of such licensed software.
My problem is, that the GPLv2's terms are incredibly strict, prohibiting any other license terms being applied by any other source in the project. This means that you can't mix GPLv2 with pretty much anything else, except the very most permissive licenses. The patent grant & protection clauses breaks GPLv2. (In another older circumstance, the CDDL had similar issues which blocks ZFS from being distributed with the Linux kernel proper. The CDDL also had a fairly benign choice-of-venue clause for legal action, which was also deemed incompatible to the GPLv2.)
So at the end of the day, GPLv2 freezes innovation and has limited my own actions because I would like to enable people who have GPLv2 libraries to use my libraries. We even have an ideological agreement -- the FSF actually recommends the Apache License 2.0! And yet I can't use it; I'm stuck with a very much inferior MIT license in order to let GPLv2 folks play in the pool.
Wait, you say, what about the GPLv3? It fixed these incompatibilities, right? Well, yeah, but then it went and added other constraints on use which are even more chilling than the GPLv2. (The anti-Tivoization clause, which is one of the more bizarre things I've seen in any software license, applying only to equipment intended primarily "consumer premises". What??)
The GPL is the FOSS movements worst enemy, in my opinion. Sure, Linux is everywhere, but I believe that this is in spite of the GPLv2 license, rather than as a natural by product. The same result could have been achieved under a liberal, or a file-based copyleft.
GPL in Support of Proprietary Ecosystems
In another turn of events, the GPL is now being used by commercial entities in a bait-and-switch. In this scheme, they hook the developer on their work under the GPL. But when the developer wants to add some kind of commercial capability and retain the source confidentially, the developer cannot do that -- unless the developer pays the original author a fee for a special commercial license. For a typical example, have a look at the WolfSSL license page.
Now all that is fine and dandy legal as you please. But, in this case, the GPL isn't being used to promote open source at all. Instead, it has become an enabler for monetization of closed source, and frankly leads to a richer proprietary software ecosystem. I don't think this was what the original GPL authors had intended.
Furthermore, because the author of this proprietary software needs to be able to relicense the code under commercial terms, they are very very unlikely to accept contributions from third parties (e.g. external developers) -- unless those contributors are willing to perform a copyright assignment or sign a contributor agreement giving the commercial entity very broad relicensing rights.
So instead of becoming an enabler for open collaboration, the GPL just becomes another tool in the pockets of commercial interests.
The GPL Needs to Die
If you love open source, and you want to enhance innovation, please, please don't license your stuff under GPL unless you have no other choice. If you can relicense your work under other terms, please do so! Look for a non-viral license with the patent protections needed for both your and your downstreams. I recommend either the Mozilla Public License (if you need a copyleft on your own code), or the Apache License (which is liberal but offers better protections over BSD or MIT or similar alternatives.)