>WoTC could sue anyone who made content for ownership even if it was made with the previous license.

Unless I misunderstand, according to the submitted thread this isn't the case.

>The good news is that, because of the reliance principle, third party creators who have already created a work under the OGL likely could continue to sell that work even if the OGL were revoked. They just could not create any new works.

Although that "likely" gives doubts.