"Bottom line, I conclude that the outlook for the US economy, US profits, and US stocks in the next 10-20 years is poor, and likely to deteriorate further."

Seven-year to twelve-year forecasts for US stocks by GMO Funds, Research Affiliates and Hussman Funds agree with you. Their forecasts average around a -2% percent real, annualized return, based on growth and valuations, and not taking into account tariffs, USD exchange rates and the other factors you mention. GMO forecasts a -4.3% real return over the next 7 years; Research Affiliates forecasts a 1.1% real return over the next 10 years, and Hussman sees a -6% nominal return over the next 12 years. Hussman provides the most detail supporting the forecast. The metric that he has found that best correlates with forward return is market cap/gross value added.

hussmanfunds.com: The Bubble - Contains the Collapse - Contains the Resurgence