I wasn't making a prediction on future returns just commenting on the historical returns used. For this case 5-7% should be more than enough as a comparison to the Tesla investment anyway. I'm curious about those risk premium estimates though. Any pointers or sources on where to read more about them?
Current expert thinking is that there is a 4% equity risk premium over cash interest rates. Current cash rates are 1% nominal or -1% real. If you are an optimist like the Fed, cash rates will rise to 3% over the long term - which gets you to 7% nominal/5% real at best.