There are some working solutions now, like NEO, Dash, and NXT. Ethereum is sure to adopt it as well; a lot of details are TBD but there's little question that the community will adopt some form of PoS.

I'm not saying it's perfect -- there are some downsides like needing a cap on fork distance in order to prevent stake bleeding attacks [1] -- but it's certainly viable.

[1] https://eprint.iacr.org/2018/248.pdf