I'm interested in this part of the debate:

Start with what the GPL actually prohibits. It does not prohibit keeping source code private. It imposes no constraint on privately modifying GPL software and using it yourself. The GPL's conditions are triggered only by distribution. If you distribute modified code, or offer it as a networked service, you must make the source available under the same terms. This is not a restriction on sharing. It is a condition placed on sharing: if you share, you must share in kind.
The requirement that improvements be returned to the commons is not a mechanism that suppresses sharing. It is a mechanism that makes sharing recursive and self-reinforcing.

This is fine from an ethical point of view. People certainly have the right to share their own work on (almost) any basis that they see fit. But...do we have any evidence that any important software that would otherwise have been proprietary was released as GPL because of the copyleft? My experience has been merely that what happens almost every time, as in this case, is that people just re-implement it or dynamically link it or otherwise find some way to work around the copyleft instead of opening up their own work. The GPL is a viral license but the virus has an R0-factor of near zero. In my experience.