The reason to use past values is because they're known. Anything anticipating the future is speculation, and relies on many assumptions. Check analyst earnings

The reason to use past values is because they're known. Anything anticipating the future is speculation, and relies on many assumptions. Check analyst earnings estimates for S&P companies for 2008 and you may be surprised what you find. I fully believe Tesla can be the number one auto maker, and get self driving working to a sufficient extent to be marketable. But what's the upside? A few X at most. To me that risk/reward is pretty awful... many more attractive companies selling at 1x PS and fast growth/smaller market cap that can go up 10x easily over a few years