> Your editor is unaware of any study showing that developers are less willing to contribute to GPLv2-licensed projects;
As I see it, the issue is that complex userspace applications like MySQL or even a larger software that would embed part of MySQL, are usually linked to a bunch of (L)GPL libraries that will eventually move (L)GPLv3. Due to the incompatibility between LGPLv3 and GPLv2, any GPLv2+ software linked to such (L)GPLv3 became GPLv3+ for all practical purpose. Example of such LGPLv3 libraries today include GNU readline and GNU MP.
As a result, GPL2-only complex userspace applications get less attractive because they are restricted in which libraries they can link to.