There's no really viable alternative for Bitcoin, unfortunately.
I'm not sure I like the proof of stake idea but maybe proof of hard drive space or proof of RAM maybe something like that could be feasible. thing is, i'm not sure those are totally environmentally friendly either since production of hard drives and memory probably has environmental consequences but maybe less than ASIC mining. but it still wouldn't be bitcoin. but i really don't think anyone is going to be able to stop bitcoin from existing using Proof of Work. that's how it was made and it can't really change from that. not even if a quantum computer appeared tomorrow that could break sha256 bitcoin would just be dead.