After deep losses last year, an AI boom has driven the stock market to a rebound in 2023. The biggest winners have been the mega-caps, companies like Alphabet, Microsoft, and Nvidia, whose size, market share, and exposure to AI have attracted investors in droves. The result, according to investing legend Jeremy Grantham, is a decidedly unstable situation, reminiscent of past stock bubbles. Grantham, who is the co-founder of Boston-based GMO, a firm that has ~$58.8 billion of assets under managem