> Mullenweg's argument that the norm for projects in "his" predicament to simply relicense to non-open-source terms

Except Mullenweg has zero authority or ability to do so, without the agreement of all the GPL contributors. Mullenweg might argue this, but he also has been around long enough to assuredly know that this is neither the case, nor possible for him to do so. Apropos of anything else, he built WordPress by forking b2.