Right, wpSEO is not executable on its own. But this alone does not automatically results in the GPL.

The question is: what are the consequences if I don’t put my plugin under the GPL v2 or v3? And the consequence is (unfortunately) only that the owner of the copyrights to the WP interface to which the plugin docks (the WP Foundation) can prohibit the further distribution of this plugin.

Contrary to popular opinion, it is not the case that the plugin is automatically under GPL. This is and remains (at least according to German law) still a matter for the plugin originator (or his legal successor etc.).

Ergo: The further distribution of the wpSEO plugin under a non-GPL license could be prevented by the WP Foundation, but the plugin is currently not under GPL (and probably was not under GPL during Sergej’s time).