The criticism's in this post really make very little sense. What does that mean that "private keys only have value when you hold them." Private keys are worthless, they only give you access to the wallets that they unlock.
Also, no, you are not guaranteed to stay at 51% forever. Vitalik has explicitly said that if a 51% attack happens, the community is expected to fork, and all of your funds will be isolated and slashed in the fork. If you want to attack again, you then need to collect another 51% of funds. This gets exponentially more difficult.
> What does that mean that "private keys only have value when you hold them."
What he's saying is that people would sell their old private keys and an attacker could then use these to create a fake history of stakers- The existence of this attack is a valid criticism against POS, though one that can be easily prevented by just syncing to the network at regular intervals.
Also, no, you are not guaranteed to stay at 51% forever. Vitalik has explicitly said that if a 51% attack happens, the community is expected to fork, and all of your funds will be isolated and slashed in the fork. If you want to attack again, you then need to collect another 51% of funds. This gets exponentially more difficult.