You Can Prove Anything with Statistics

One reason for the dispute is that there's a fairly clear commercial motivation behind the statistics that are being used to support deprecation of the GPL. They come from companies that sell assurance and analysis services for open source, and who promote "license compliance" as a risk for developers. They have a vested interest in developers fearing reciprocal licensing as they aim to monetize the amelioration of that fear. Look for pretty much any scare story about "license compliance" and it will somehow be connected with what one speaker called the "compliance-industrial complex."

The complexity arises from the subjectivity of phrases like "a decline in the GPL." Even when linked to data apparently supporting the statement, underneath is an assumption that the popularity of a license is demonstrated by the metric being cited, such as the number of new projects on GitHub using non-reciprocal licenses. Anti-GPL articles almost never justify why their popularity metric is valid.

Certainty & Safe Spaces

The open source license the project uses defines and guarantees the things over which those developers need certainty. All give the right to use the code for any purpose, to share it with anyone, and to make whatever changes they want. Beyond those essential freedoms, different communities need different certainties.

Communities working on code that is normally used directly, alone, and in its entirety – application software such as LibreOffice, for example – may well want their license to also guarantee reciprocal grants of the same rights to other developers. Most of the LibreOffice core developers work for companies that offer support, training, customization, and deployment. Because everyone who offers these services has to contribute their work as a license requirement, it's much less likely that a freeloader will be able to undermine their business.

For developers mixing ingredients from multiple origins – frameworks, components, libraries – reciprocal license requirements increase the uncertainty rather than decrease it. Their employer may be concerned about managing the different reciprocal duties of different licenses, such as the Eclipse Public License (EPL) and the GPL. For these developers, it is much simpler to use non-reciprocal licenses for their code, especially if that code is not directly monetized.

Horses for Courses

Neither of these approaches are the One True Way. Both have their place, and both have substantial, growing international acceptance. All the same, the growing acceptance of open source for corporate use – some would now say "dominance" – could easily appear as the only trend to those whose gaze is fixed in that direction. There is no doubt that the growth of corporate open source development has seen new code aimed towards it tend to use non-permissive licenses.

So perhaps the best way to view the subject is to note that the open source world has grown enormously. The use and support of the GPL has also grown with it, but new strengths have also emerged related to corporate adoption of open source. Deciding which is dominant may well be a matter for your ideological biases rather than an objective absolute!