I totally get the skepticism about using past investment results to predict future investment results. The world has changed a LOT since the distant past, and there is reason to believe it will change a LOT from today's status quo. At some point it is fair to ask what returns on U.S. railroad stocks in the 1800s have to do with where the Nasdaq will be in 2033. Things that have changed include technology, how currencies are managed, culture, the ease or difficulty of investing, the ease or difficulty of transferring funds, banking, competition, communication, availability of natural resources, human lifespans, political stability, liquidity, taxes, the legal environment, and a Federal Reserve with a mandate. Add to it some selection bias: People in most countries could not retire on a SWR of 4% because most countries have experienced dramatic instability over the past 150 years or so. We in the U.S. tend to be talking about U.S. historical data, when the U.S. has happened to be one of the most stable markets in the world. Recall that the U.S. had a coup attempt just a couple of years ago, and has sufficiently weak institutions that we could go the way of Turkyie' as soon as the next elections.
So using only forward-looking rationales, here are reasons to think recent patterns of returns will continue:
1) Inflation expands margins. Example at 5% inflation:
Year 1 cost of production: $1, revenue: $1.50, margin: $0.50, stock price @ PE=15: $7
Year 2 cost of production: $1.05, revenue $1.575, margin: $0.575, stock price @ PE=15: $8.625, a 23% increase.
So as inflation occurs, the nominal value of corporate earnings increases with inflation, which is then multiplied by the PE ratio. In the simple example above, the company faced a 5% increase in costs, raised their prices 5%, experienced a 5% increase in margin, and had a stock price increase of 23%.
2) New products expand economies. Think about all the economic growth that came from things you couldn't buy 40 years ago, like computers/smartphones, cars that last 250k miles, most pharmaceuticals and medical procedures, streaming services, internet services, software and SaaS, exchange traded funds, options, robots and drones, wrinkle-free clothes, induction ranges, GPS, audiobooks and podcasts, well-insulated housing, pet insurance, solar panels, etc. How much economic growth over the past 40 years came from the development of products no one could have imagined back then? Now think about the next 40 years and ask what we can't yet imagine.
3) Developing economies will continue to develop. The big story of the last 50 years was China and Eastern Europe transforming themselves from backward, impoverished places where children occasionally starved into vibrant economies with fast-rising standards of living. China's development boosted US corporate profits by offering retailers and transporters larger margins on cheap imported goods, by pushing down US inflation and enabling lower interest rates in the US, and by enabling the rapid release and production of new products. Now think about all the other formerly developing countries that are getting their shit together and emerging from poverty, like Chile, Brazil, Columbia, Mexico, Philippines, Indonesia, Thailand, Vietnam, India, Nigeria, and more. There's a definite pattern of development which could in coming decades slowly extend across Africa, more Latin American countries, more of Southeast Asia, the Middle East, and perhaps a future democratic Russia. As the worldwide economy gets bigger - much bigger - so will the opportunities to invest in new, fast-growing places.
4) Technology and globalization will continue to drive productivity upward. The big stories of the last 50 years were globalization and computers. Globalization allowed for the more efficient matching of human capital and financial capital, which both pulled hundreds of millions of people out of dire poverty AND increased everyone's financial capital. Computers meanwhile greatly simplified information processing (e.g. corporate accounting offices) and communication (email, mobile phones, video calls). The big story of the next 50 years might look like AI tools that design molecules to cure cancers and defeat viruses, design manufactured products to optimize their performance, invent new industrial processes or solutions to old problems, instruct robots to do work for us, enable mass production of customization, entertain us, or design the next AI tools. This will drive up productivity, as less human labor will be required to produce all sorts of services.
5) Stock markets automatically sort out successful ideas from the unsuccessful ideas. Companies that fail to produce value eventually fall out of the indices and become penny stocks, or never make it into the indices in the first place. The companies which do the best jobs of creating value for their investors come to dominate the indices. Thus, if you buy into an index fund or any value-weighted fund, you are buying the outcome of a selection process that has already occurred. As you hold your index funds, the indices adapt to changing circumstances and displace the poorest performers with the new best performers. Thus, while it might not be clear how any particular company is going to perform for the next 10-30 years, it is clear that in 10-30 years your shares of SPY or QQQ or VTI will reflect the best ideas in the modern economy at that time.
6) Pinker's "Better Angels" are leading to growth. Author Steven Pinker created a stir among pessimists with his 2011 book, which amassed a vast pile of data supporting the conclusion that humanity is on a centuries-long trajectory of becoming less violent, less criminal, and less warlike. In economic terms, it has become more lucrative in modern times for an ambitious executive to start a corporation rather than to become a warlord. This is a departure from millennia of human history in which the best way to get ahead was to steal wealth and land from other humans in a zero-sum game of violence. Pinker makes a strong case that people worldwide are becoming more civil, better educated, and more inclined to democracy. There are exceptions, but they are getting rarer in the long view. The inclusion of women in economies around the world is in itself reason to expect faster economic growth - like the US experienced when it included women in the economy during the 1970's and 1980's. So if you think of certain regions as hellholes of war and poverty, imagine that not being the case in a couple of decades, and imagine publicly traded corporations making trillions of dollars in such places.