Note: To be fair: I do recognize that validators believed they were actually protecting the network from an attack by coordinately installing 0.5.19 of classic-core, and have been cautious in going forward.
Here is an explanation of why these were not actually an attack. In addition, in terms of Byzantine fault tolerance (BFT), this is the very idea behind not only blockchain (to optimize against it), as well as Tendermint ( What is Tendermint | Tendermint Core ), which underlies Terra.