9.7% (or maybe 0.8%) is the average over the last 90 years and I don't think there is any other data points you could reasonably use to say it won't continue like that over a long enough time frame.
Well, over the last 90 years the USA moved from important regional power to the economic and military hegemon of the world. That's quite the wave to be riding as a US stock market investor. It's also quite a hard act to repeat.
To maintain the historical growth rate, you have to believe one of the following two things, or some combination of them:
1. The US will continue to amass a larger share of the world's wealth, indefinitely.
2. The economy of the world at large will begin to also grow ~10% a year, a number far in excess of the historical average or any well-informed estimate.
Sure there are; valuations are considerably inflated now compared to the average (or starting point) of the last 90 years. Percentage earnings and dividend rate are both considerably lower. The long term expected return for the market is therefore also lower.
