The original plan would have been hobbled by technical debt. They were going to implement the core algorithm as a smart contract on the old chain. It would have supported a much smaller number of stakers and shards.

In mid-2018 they threw out that plan, choosing instead to implement PoS as an entirely separate chain, integrated with sharding. This entailed a delay of over a year, but it meant a minimum stake of 32 ETH instead of 1500, and an overall much more efficient and scalable system. There are three main phases, the first being the core proof-of-stake "beacon chain," which is currently running a multi-client testnet.

There have been various proposals for migrating legacy Ethereum into all this; here's the latest: https://ethresear.ch/t/alternative-proposal-for-early-eth1-e...

Cardano is PoS and released its testnet last month.

Tezos is another.

Cosmos is PoS and released its mainnet in March.

A Proof of Stake consensus for a public blockchain like Ethereum has different requirements and is substantially harder to execute than for a permissioned network.

Definitely agree, although it’s important to clarify that Cosmos is not permissioned. Anyone can start a validator and join the validating set.