What if some random billionaire bought that much stake off the market and publicly stated that he was going to destroy Steem

dump it all to him and others following the market, because buying 50-70mil steem on the market would push the price really close to that moon.

witnesses WOULD do this to some random community member, it's the fact that they CAN.

they can also just implement a code and crash steem. they can do anything if supermajority consensus is reached. Anything. So we vote for people that we think will not do stupid things, and if they did they would most probably not be in the top 20 anymore.