There are actually lots functional proof of stake coins like byteball and NEM for example (but I share your skepticism about long term viability of proof of stake BECAUSE it doesn't cost anything). I think the energy "waste" isn't waste because it serves a purpose. For someone to forge a transaction they would need to control a huge amount of hash power which would cost a fortune. On the other hand, there's always ideas like gridcoin that use that hash power to do scientific computations at the same time (best of both worlds?).