1. Vanguard forecasts 4%-5% annual returns for U.S. stocks over the next decade. 2. AI's impact on the economy doesn't guarantee stock market gains. 3. In Vanguard's bear case, returns could drop to negative 2% to 2%. 4. Market skepticism grows over Big Tech's ability to maintain dominance. 5. Value stocks may outperform growth stocks if AI leads to productivity gains.