Don’t necessarily take IPOs as a signal; the early IPO is not as attractive as it once was, for a wide variety of reasons. Companies with no product taking hundreds of millions in private funding should arguably be seen as the modern equivalent.
There is a question of what is even a bubble. John Cochrane did a study that showed that the value of Amazon alone justified tech stock index valuation even in 1999.
What is a "bubble" even? For me it would be self-driven cycle of upward valuation whose fundamental value never justifies the market cap in it. An industry that rises in price just a few years later to meet and then far exceed the previous valuation does not need to be a bubble. A bubble is not just a high valuation that decreases in price sometime in the future.
I think one of the core features of a bubble is that the median market participant loses money. I think that definitely occurred in 1999. I don't think it has occurred in 2024.
I'm not convinced it will happen either. Venture Capital operates on a business model that is really hard for human intuition to deal with. 1/1000 success rates require so few break out successes that, yes -- they can fund an entire sector that will, with extremely high probability, fail and still be profitable.
There have been plenty of these retroactive reviews of the 1990s tech boom that have questioned whether it was a bubble, by changing the definitions that everyone else uses to define valuation ratios and bubbles. I've heard it argued that the dotcom bubble wasn't a bubble, because the valuations were justified in the moment due to future expectations, until they weren't, which just sounds like the definition many others use to define a bubble.
As to Cochrane, even if the value of Amazon is argued to have justified the tech sector valuation, the fact was that the tech sector valuation was not all, or even mostly, concentrated in amazon, which is why it was a bubble.
But we’re not peak AI yet in my opinion. Companies aren’t IPOing at dizzying valuations with just an idea and a few html developers.
The vast majority of the AI boom is centered around companies that were successful before the boom. Apple. Nvidia. Microsoft. TSMC. Google.