At one point this year, the S&P 500 (^GSPC -0.11%) looked like it was in trouble, falling below 6,400. There's just no denying that the index is expensive these days. Back then, it was internet stocks surging to obscene valuations. What the optimists will say is that, unlike internet stocks that made no profits, AI companies are generating significant growth. Plus, it wasn't just risky internet stocks that crashed during the dotcom bubble.