What are you trying to say here? Coin weight in a Proof of Stake type model only deterministically gives you control if you own more than 50% of the coins.
I don't have time to reply to all this right now, but it's pretty common knowledge that you only need 51% of coins
currently staking to control the network and
not 51% of total coins. Using coin age as a finite resource in this system just makes it easier to attack, which is why NXT never used it, and why Blackcoin is removing it. Reputation makes a much better finite resource than coin age and makes it so large holders can't perform effortless attacks, just like how PoW works.
For simplicities sake it makes sense to assume most people have an interest to stake their coins or participate with their resources but yes, you are right that you only need > 50% of coins staked to deterministically form the longest chain.
It makes no real difference because whatever model you choose always reduces down to how likely it is for a single
active entity to produce > 50% of the blocks.
There is very little real incentive to perform a > 50% attack because you'd shake confidence, no matter what method is used for block creation.
I think the issue is greatly exaggerated because then it is much easier to promote an altcoin with a new mining/minting scheme.
To me the real problem lies in ensuring privacy. And I do not mean through a new altcoin but rather implementing stronger privacy in bitcoin itself.