While US stock markets experienced a brief shock yesterday with the news of the government shutdown, OpenAI's $500 billion valuation sent the indices to new highs.

The market is seemingly being fueled by the AI ​​wind, and investors are watching its power without question.

So, is all this activity truly sustainable, or is a bubble forming in the most expensive stock market in history?

The Buffett Indicator provides a striking answer to this question: the total market capitalization of US stocks is close to twice the size of the economy.

Compared to historical benchmarks, the ratio before the dot-com bubble in 2000 was 150%, and before the 2008 crisis, it was around 110%. Today, it stands at a record high of 217%.

As Warren Buffett puts it, this is "playing with fire."

But is it really that simple?

Technology giants like Nvidia and Microsoft, which will dominate the stock market in 2025, don't solely generate revenue from the US.

The majority of their revenue comes from global markets, making it difficult for the Buffett Indicator to be a direct signal as it was in the past.

So, while the indicator alerts us to risk, today's digital and global economy may not fully reflect the extent of this risk.

However, it would be wrong to say there is no risk. With interest rates remaining high and the Fed taking cautious steps, the possibility of a sharp short-term correction in such an expensive stock market is significant. While there isn't the rapid selling pressure of the dot-com era, investors must now evaluate the risk-return balance more carefully.

So, what should investors do?

First and foremost, balancing the portfolio is critical. A weightlifting strategy offers an opportunity to limit risk by holding a combination of stable, long-term dividend-paying stocks and high-growth technology stocks.

Furthermore, simply looking at P/E ratios isn't enough; Indicators such as Shiller CAPE, the CAPE-to-return spread, and the PEG ratio allow investors to evaluate both the narrative and the profitability.

Finally, caution is necessary when following the trend. The 2025 rally, centered around artificial intelligence, megatechnology, and data centers, is fueled by liquidity and narrative.

But history shows us that reality eventually reflects in prices. Berkshire Hathaway's recent cash buildup is a classic example of not acting hastily and achieving healthy returns over the long term.