Because you think of buying the stake... Many exchange owners had a stake history with percentages much higher that. So if an exchange gets hacked the coin gets attacked?
Unless the exchange has more than 50% of all coins, or is the only node, no.
Doesnt look like a sound monetary system to me. You can buy old stakes keys to attack. You can even create StakeUndo service like
http://www.bitundo.com/ and gain at no cost for old stake owners.. Market cap has nothing to do and valuation does not improve or reduce the security model
Everything here is false and I already explained why, in the second half of my first post. I'm not replying to you anymore until you start reading what I already wrote on the topic.
The fact that you gave some random percentages earlier does not qualify for an answer. current forging is around 20% and anything greater than that can be a source for attack. Giving random percentages does not change current facts.
Also the current 20% will be less when more rational forgers (excluding the three enthusiasts controlling 50% of current stake forging) who will prefer to put the coins in better use... i.e. deposit them in a just-dice.com type of service to earn more. If such services/stock markets compete for capital you should expect the effective % stake for forging to be much much less