Was wandering the same thing. We have several positives and several negatives weighting against each other when it comes to market performance in the coming decades:

Positives:

• AI advancements in particular.
• Several streaks of technological advancement in general.
• Accelerating returns speeding up the innovation cycle.

Negatives:

• IQ decline due to dysgenics. Smarter people are having less children since the Industrial Revolution.
• IQ decline due to changing ethnic proportions. Headcount in the north is diminishing.
• IQ decline due to aging population.
• Declining health and fertility due to dysgenics and lifestyle.
• Low fertility leading to stagnation in headcount.
• Decline in productivity due to aging population.
• COVID-induced hysteria causing economic turmoil.
• Geopolitical instability.

If for some reason technological progress stalls in the coming decades, we will have an aging, dwindling, sicker and dumber population towards the end of the century, by some estimates falling to 85 average IQ, from today's 100 standard. Some infrastructure and nation states will have collapsed and balkanized into tribes led by warlords. Huge uptake in religiousness and conservatism as well. I find this scenario highly unlikely.

If I had to guess, the positives will outweigh the negatives, though we could easily see a flat decade ahead in the markets due to COVID-induced mass hysteria and the war in Ukraine. My personal strategy would be to invest about 40% in broad market ETFs, 40% in tech heavy ones, and 20% in resilient assets, such as farmland and gold. The S&P 500 has historically doubled every ~10 years in real returns, inflation adjusted. I would be surprised if the rate of doubling diminished, despite some recent pessimistic projections, as they tend to completely disregard accelerating returns in tech and science, and simply project from what happened in the past decades.

As a side note, if you invest in the stock market and elect not to have children, you are a free rider, in my estimation, as generational expansion is half of the growth equation. So have lots of them.