Which PoS coins? I'm aware of only NXT, which has much more online, but then it works in a different way. Peercoin isn't PoS, it failed, very bad design.

What much more? Taking last 100 blocks indicate something like 20% and 10%  coming from 3 accounts. Dat decentralization....


Why do you assume all these nodes will be online at the time of the attack?
I didn't wrote that. Still, even at your 10% of honest stake, as you said, you need >10% of hostile stake. That's still much more as % of market cap

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? 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  

Not everybody, just people with small amounts. First, it's not like there's only going to be one person. Second, big holders will dominate with their own nodes. Third, doing an attack like this would be actually criminal, so don't lend forging power to anonymous people in third world countries and it's going to be ok

big holders dominating does not sound like a decentralized solution to me  

Not lend coins, lend forging power. It's not required, but it makes the network a tiny bit safer
The point I made is it doesnt since it gives stake history power to few which can be used in the future


It's true that it's a problem if the loss amount is extreme. However, over long periods the validity of older blocks will be close to 100%, as many different stakes will get transacted.

I dont understand what the validity of older blocks will be close to 100% means... citation maybe?  


All PoS coins are very centralized in their stake distribution and dont forget that when you say they are decentralized. Let alone all the other reasons PoS coins are not decentralized.


PoS is just a consensus method for a currency. Distribution is an orthogonal topic

Not that orthogonal if you want a DECENTRALIZED consensus method for a currency based on PoS


It offers orders of magnitude more security, because you need a big percent of market cap, not just mining power.
No it doesnt for the arguments stated above unless you rely on checkpoints