What are the problems with current PoS systems? Not being glib, seriously interested as there are at least a few that implement some kind of Pos (for example dPoS with Ark)

Permissioned networks are a very much easier setting to design a consensus protocol for than public ones.

Describing a consensus algorithm as "Proof of Stake" simply means it uses some kind of stake to secure its decisions, this doesn't say anything about the assumptions it starts from.

Existing "PoS" blockchains don't have the same decentralization goals as Ethereum.

> Existing "PoS" blockchains don't have the same decentralization goals as Ethereum.

What about Cardano and Tezos? aren't they decentralized PoS blockchains?

And Cosmos

“Existing ‘PoS’ blockchains don't have the same decentralization goals as Ethereum.”

What makes you think this?

How is Ark less decentralized?

From my understanding, the issue with PoS is that if a chain forks you can mine both sides without penalty. A consequence of this is that if someone wants to retroactively create a fork that starting from a block a few years ago, then people could do a few years worth of mining of this fork in an instant. And it wouldn't be possible to tell these two chains apart cryptographically. With PoW it would be computationally expensive to pull such a stunt.

Not exactly, it's here and technically working in multiple production blockchains, it's just less secure than Proof-of-Work. So it depends what your definition of "works" is.