I still don't see how a nothing at stake attack could succeed.
Maybe if you read my PoS paper where I actually give a specific attack?
Buying majority of PoS coin supply isn't exactly nothing,
It isn't exactly anything, either. "majority of PoS coin" is not well-defined in the absence of consensus.
and finding private keys of the initial stakeholders does not help if you have checkpoints.
...yes, obviously you can create a non-distributed consensus. Humans have been doing this since before we had language.
Nxt uses a reorg window of 720 blocks. blocks older than that won't be accepted by any client. This means that checkpoints are set up in decentralized manner (each client sets its own reorg limit). You need to effectively buy 51%.
Is this actually what they do? Reorg windows simply make forks permanent. There is literally no attack they are capable of mitigating -- either you have no deep forks and they are pointless, or you do and they result in permanent partitioning of the network. (This idea has come up hundreds, if not thousands of times, and is orthogonal to the consensus mechanism.)
I would like to see a nothing at stake attack succeed,
Stake-grinding is an example of a NaS attack. See peercoin or the original NXT for examples.
so far I only saw 51% attacks on low hashrate PoW coins. Also I don't see how decentralized consensus should not be possible in PoS?
Maybe if you read my PoS paper?
I see it working in real world while the "consensus is not possible" statement is theoretical.
I see this claim, along with its variant ""consensus is not possible" statement is bullshit", a lot. But this paper has been out for over six months, has been read by thousands of people, has changed the discourse around PoS to the point where I was accused of
strawmanning after its last appearance on Reddit since "nobody is actually proposing distributed consensus by PoS", and yet there have been exactly zero counterarguments. I'm getting tired of these sorts of proudly uninformed comments.