More recently Vitalik has said Ethereum PoS is at least a couple of years away, likely due to new attack vectors that require further research https://arxiv.org/abs/2110.10086, one of which enables an attack with only 0.09% of the total staked. It would have been foolish to fork knowing attacks like these are still being discovered.

"with more than 99.6% probability, an adversary with 0.09% of total stake is in a position to execute a 1-reorg for any given day."

Another which could prevent the chain moving forwards indefinitely.

"an adversary controlling 15% of stake can stall PoS Ethereum"