Why would the network without human intervention not accept my second chain? It perfectly follows the rules...
Because you cannot reverse public blockchain using your one secret longer blockchain twice.
The point is that you dont need to reverse it twice but only once.
Apart from that you misunderstand what a decentralized autonomous corporation like bitcoin means. There are certain rules and all clients follow those rules. The rule is to work on the longest chain aka most cumulative stake for PoS and most work for PoW.
If you have the stake in your history you can reverse it 2,3,5 and as many times as you like if you want. Noone stops you from that only human intervention aka centralized checkpoints.
Why would the network without human intervention not accept my second chain? It perfectly follows the rules... The other attack you are describing is the nothing at stake problem where you can keep trying double send without any cost. I am talking about a more severe attack that will force the network to become centralized or investors/exchanges/reputation take massive losses....
It do cost to attack Pos coin. You have to buy 5% coin from market.
You only need to own the keys at some point in the past. For example, a person who deposits/withdraws multiple times from exchanges (think of exchanges using multiple addresses) can get the keys for a lot of stake ownership in the blockchain history and use it to create an alternate chain. Exchanges own a lot of stake in their history... Pools do the same.... and many other services...
I am talking about a more severe attack that will force the network to become centralized or investors/exchanges/reputation take massive losses....
Please describe the more severe attack in detail.
https://en.bitcoin.it/wiki/Altcoin#Proof_Of_StakeIronically, Feathercoin, AudroraCoin and Quarkcoin, all of those PoW coin, have centralized checkpoints.