I’ve been playing around with Gemini a bit and asked how high it estimates the probability that the S&P 500 will not be higher in 5 years than it is today. FWIW, here is the answer:

Based on historical data and current market models, the probability that the S&P 500 will not be higher in 5 years (February 2031) than it is today can be assessed in a differentiated manner.
Statistical Probability
Historically, a 5-year period is usually profitable, but extreme overvaluations shift the odds:
Historical Average: For any random 5-year window, the statistical probability of a loss is only about 10%.
Valuation-Adjusted Risk: In market phases where the Shiller P/E (CAPE) was above 35-38 (currently approx. 40.36), the probability of a "lost decade" or stagnant 5-year periods increased massively.
Historically, such levels were often followed by nominal returns of only 0% to 3% per year.
Why a Sideways Movement is Likely
The combination of the Buffett Indicator (approx. 224%) and high market concentration leads to the following forecasts:
Low Expected Values: Institutions like Goldman Sachs forecast a nominal annual return of only approx. 3% for the next 10 years. In a 5-year window, a single moderate recession or a mere return to
average valuation ("Mean Reversion") can be enough to completely eliminate these gains.
No "Cushion": Current valuations leave no room for error. If earnings growth (currently optimistically expected at 10-14% for 2026) disappoints, a correction to levels seen in 2021 is a distinct possibility.
Probability Estimate
Including the extreme overvaluation (2 standard deviations above the trend), the risk of a negative or flat real return over 5 to 10 years is significantly higher than the historical average.
Nominal estimate: The probability that the S&P 500 will nominally not be higher in 5 years, due to the current "Priced for Perfection" situation, is estimated at 25% to 35%.
Real (inflation-adjusted): The probability that you will not have gained any purchasing power after inflation is even higher and could be over 50%.