Well there are several altcoins using proof-of-stake. Whether they'll be as secure in the long term is an open question, but I wouldn't go so far as to call it a perpetual motion machine.
Re:The Bitcoin Blocksize: A Summary
by pstrateman on Wednesday August 12, 2015 @02:17PM
(#10049580)
Proof of Stake does not work.
Yup. But I wouldn't hold my breath. No one has ever created a decentralized consensus algorithm with the properties bitcoin has, before or since. And since bitcoin achieves its unique properties as a result of the economic costs of mining, asking for a mining-less bitcoin is kinda like wishing for a perpetual motion machine.