51% attack means To own the 51% of hashing power over all not 51% of btc
When a coin is POW, yes, but POS is different. POS is vulnerable to a 51% shareholder attack. Honestly I don't believe that this could be done. Good luck finding 7 million Bitcoin to buy.
You have it wrong. You don't need 51% of the total coins to do the attack. You just need 51% of the total
staking coins to do the attack(which is much lower than 51% of the total coins).
Thats why PoS is not as secure as PoW.
That's a good point.. though that seems to be alleviated by things like
"balance leasing" "it is possible to loan forging power from one account to another without giving up control of the tokens associated with the account. Using a transaction of the account control type, an account owner may temporarily reduce an accounts effective balance to zero, adding it to the effective balance of another account. The targeted accounts forging power is increased until the end of a time period specified by the original account owner, after which the effective balance is returned to the original account."
https://wiki.nxtcrypto.org/wiki/Whitepaper:Nxt#Balance_leasing