The Century-Old Engine of Survival of the Fittest: How the "Metabolism" of US Stocks Forges the World's Strongest Bull Market

Whenever investors ask, "Why US stocks?", the most common answers are: strong corporate profitability, leading technological innovation, and the stability of the US dollar system. These are all true, but they are superficial. The underlying engine that has driven the sustained bull market in US stocks over the past century is the market's powerful "metabolism"—high-quality companies are rapidly brought to the forefront, laggards are systematically removed from the stage, and the index maintains vitality through continuous self-renewal. Only by understanding this mechanism can one truly grasp US stocks. A Century-Long Corporate Selection Process The S&P 500 index, established in 1957, is recognized as the best window into the changing structure of the American economy. However, if one were to pull out the original list of 500 components from that era and compare it to today, a shocking fact emerges: fewer than 20% of the original members have survived. The vast majority have either been acquired, delisted, or removed from the index due to scale shrinkage. This is not an accident; it is the norm of US stock market operation over decades. Kodak was once synonymous with "American imaging" and filed for bankruptcy protection in 2012. Sears was once a flagship of American retail but faded from the historical stage in 2018. General Electric, which once ranked among the top global market caps for a long time, was also removed from the Jones Industrial Average in 2018. In contrast, new-generation technological forces like Nvidia, Tesla, and Broadcom have recently taken significant positions in core indices. Every year, the S&P 500 conducts several component adjustments, replacing declining entities with representatives of the new economy, ensuring the index remains in sync with the times. Research institutions have calculated that the average lifespan of large listed American companies has shortened from over 30 years in the 1960s to under 20 years. This sounds harsh, but it illustrates a simple truth: no position is eternal, and no winner can rest on their laurels. It is this pervasive competitive pressure that forces listed companies to continuously invest in R&D, optimize governance, and enhance shareholder returns. The Two-Way Gate of Entry and Exit: Three Institutional Pillars Open Listing Channels US stocks operate under a registration system, where the market itself judges whether a company can list and at what price, provided that information disclosure is adequate and compliant. This allows new economy companies to connect with the deepest global capital pools early in their growth. The speed at which the capital market accepts innovation is significantly faster for companies like early Microsoft and Amazon, and recent AI infrastructure firms, compared to most other markets. Corporate financing efficiency is high, leading to faster innovation implementation, which in turn increases the supply of high-quality assets feeding the secondary market. Rigid Delisting Constraints The delisting standards in US stocks are clear and strictly enforced: companies face delisting risks when metrics such as stock price, market capitalization, number of shareholders, and information disclosure compliance cross certain thresholds. Over the past three decades, the number of domestic listed companies in the US has decreased from over 8,000 in the mid-1990s to around 4,000 today, establishing a normalized balance of "many listings and many delistings." It is worth noting that delisting does not mean disappearance—many companies are reorganized through bankruptcy restructuring or are revitalized after privatization, allowing market resources to continuously flow to more efficient users rather than being occupied by inefficient assets for too long. Active Mergers and Acquisitions A mature intermediary service system and a tolerant M&A regulatory environment result in extremely high integration efficiency among American companies. Underperforming companies are acquired, redundant businesses are dismantled, and inefficient assets are repriced. These continuous transactions constantly "cleanse" the existing market supply, injecting new growth momentum into the indices, and preventing valuation bubbles from accumulating long-term. How Metabolism Translates into Long-Term Investor Returns For the average investor, the value of this mechanism can be summarized in one sentence: index funds perform the culling for you. Buying an index fund that tracks the S&P 500 is equivalent to entrusting the work of "eliminating the weak and embracing the winners" to the rules themselves—underperforming stocks are automatically excluded, rising leaders are automatically included, and investors do not need to personally judge which company will survive the next decade. This also answers a classic question: why is the long-term strategy of "simple holding" broad market indices repeatedly effective in US stocks, while long-term holding in many markets lacking an exit mechanism can easily lead to a "value trap"? The answer is not that investors are smarter, but that the market structure itself is different: one is flowing water, and the other is stagnant water. When the market can continuously self-cleanse, time becomes an ally of quality assets; when the market can only take but not release, time tends to dilute the quality and returns of the overall portfolio. Looking further, the process of culling has shaped the distinct sector transition history of US stocks. In the era dominated by industry and energy, index weights were occupied by steel and oil giants; after the rise of information technology, weights rapidly shifted toward software, semiconductors, and the internet. Sector rotation is not merely a speculative phenomenon but a projection of economic structural evolution onto the capital market, and US stocks are precisely the clearest and fastest-updating market for this projection. For investors, tracking changes in the index structure is almost like receiving a free "health checkup report" of the American economy. Three Insights for the Average Investor Core positions should be entrusted to broad market indices. Since indices possess an inherent "blood exchange" function, average investors are better off using broad indices like the S&P 500 or Nasdaq 100 as ballast for overseas asset allocation, letting the rules manage the risk for them, rather than betting on a single stock. Do not promise "forever" to any company. The speed of decline for former giants can be far beyond imagination. The prerequisite for long-term holding is the continuous validation of a company's fundamentals, not brand memory or emotional preference. Regularly reviewing the investment logic is a fundamental skill for navigating cycles. Treat delisting and restructuring as the norm, not a black swan event. Choosing US stocks means accepting its "fast forward and fast exit" ecosystem: this is both a source of risk and an engine of returns. Diversified and portfolio-based investment discipline is a prerequisite for mastering this mechanism. In conclusion, the greatness of US stocks has never been about how many companies successfully reach the top, but about the fact that those who reach the top constantly face challenges, and those who fall exit the stage in a dignified and orderly manner. It is this almost ruthless rationalism that ensures the entire market is always composed of the most vibrant companies, turning "long-term upward movement" from a wish into a mechanism. For global investors, this may be the deepest and most memorable answer to "why US stocks."