...steem would have a hard time recovering from a shutdown of the top witnesses.
Actually that is a non-issue. If a top witness goes down there is a big list of backup witnesses that can fill the void in a short period of time.
A real concern would be if the witnesses have there nodes setup on the same vps providers. In that case the blockchain can be crippled if let's say a government targets the data centers where the servers are housed.
Because the majority of the hashing power of the mining pools for Bitcoin & Ethereum comes from china both coins are not laying on a bed of roses. It would just take a political decision from the Chinese government to shutdown the mining farms (or worse bribe them or coerce them to act for their own selfish interest)
I understand your point. This is why I am a bit reluctant to use privex. I don't know how many witnesses use them.
What I meant was more like say the 40 was down. Are the rest that ready to take over and support this charge? I am not so sure. Also witnessing means having one node doing the job so more chance to be taken down. Actually, as a witness not in the top, it makes sense to attack and try to deactivate the other witnesses.