That is true only to the extent that proof of work has already been performed. You have to stake something of value or POS doesn’t work. Staking a POW coin to “mine” a new POS coin works. So does burning a POW coin to convert it to a new POS coin that can be staked for more. But, you can’t bootstrap a POS coin without leveraging proven investment from previously burned work.

At this point Bitcoin and Eth have about a trillion USD of built up burned work. Is that enough to prime the pump for a POS-based world economy? I dunno. Probably?

Does this mean that Cardano (ADA) shouldn't work, because it's Proof of Stake from the beginning?

That's true. So, where are those currencies in the market capitalization charts and news articles?

Remember; the last time Bitcoin tried to change its mining algorithm, the democracy voted for a split. Ethereum has said they're moving from Proof of Work to Stake; I'm not surprised, given the Ethereum developers seem to abhor Work in all of its forms, including making progress on Ethereum itself, so we'll see if that actually happens in a few decades.

PoS is a theory right now, with some minor market traction, but nothing to the degree of BTC or ETH. Don't levy it as a promise which absolves the environmental sins of cryptocurrency, when its so obviously unlikely to do so. Bitcoin is the zeitgeist; its actively destroying the planet; its not moving to PoS.

Ethereum's PoS has been running in production for three months, and has about $5 billion staked. So far it's running perfectly.

They still have to migrate the rest of the network to it, but that's a relatively easy task. Once you have consensus, adding more data to reach consensus on is basically just adding a hash value to each block.

No theory to backup this claim. We had what is now rebranded PoS well before Bitcoin and it never and never will solve the problems PoW solves