This is why many coins are moving from mining to PoW, it's already a solution that is being implemented and the results will show in the coming months and years. Most of the coins shy away from mining due to the many other options that are currently available, at the begin it was necessary for the technology because there wasn't any other options but as people are developing the technology they find different ways to validate transactions. PoS is only one of many examples out there but it will be already implemented by many major projects which is really positive. Others will follow in due time and in the future we won't need mining to start with, so the current electricity waste is just something that we have to deal with until better solutions will be available for the market.