Maybe I'm missing something here but this summary feels pretty FUD-y. WotC has the right to re-license their own content, and we are free to disagree with that decision, but surely they have no legal means to force
other creators to switch their own content to the new license version simply because they are using a license text that was drafted by WoTC? Plenty of OSS projects have stuck with GPLv2 in similar circumstances.
And I'm less clear on this point, but are they even able to change the license terms under which third-parties have used WotC content that was already published under the old license? Couldn't these third parties continue using the existing content so long as they forgo new additions which are published under the more restrictive license? Or can they really revoke that licensing on previously published content?