Vandalism is always reverted on SO, even if done by the original author. No knowledge is lost. Suing OA for violating the CC-BY license might be possible, but I’d wager SO is not interested in suing them, and since they hold the rights, not much can be done by others.
I fully understand why they are doing this, but we are just losing a mass of really useful knowledge. What a shame…
Vandalism is always reverted on SO, even if done by the original author. No knowledge is lost. Suing OA for violating the CC-BY license might be possible, but I’d wager SO is not interested in suing them, and since they hold the rights, not much can be done by others.