Yes, PoS has been running safely for 1-2 years. (Ethereum isn't the first working PoS chain but it will be the largest one by far.)
There's a massive difference in the decentralization of Ethereum's PoS implementation vs. Cardano, which uses delegated PoS.
If only there were other PoS chains.
How so?
Ethereum uses signature aggregation to allow for a very large number of full-fledged staking nodes. Most PoS chains have a small number of full-fledged nodes, and everybody else delegates their stake to one of those.
Yes, it’s been running for years on many other chains and most new chains have adopted PoS (I can’t remember the last time I heard about PoW in state of the art blockchains).