AI is here to stay, but the valuations are likely way, way too high. OpenAI may survive, but their investors may not. Nvidia's stock will likely tumble, which will bring down the S&P 500, etc.
But when you think Google did ~$265B in ad revenue last year (a large portion of their ~$350B in total revenue) - I think $500B is probably about right for OpenAI.
ChatGPT has 800 million users today. Not crazy to bet on that being 1B within the next couple years. If they turned on ads for that product alone the advertising revenue would be immense. They also have so many other avenues to find and generate revenue from (partnerships, FDEs, their PaaS).
I can see why investors are still piling in at $500B. The bets are 1) The costs will come down 2) they will start to capture a lot of revenue from their products and services 3) their consumer products will continue to grow.
None of these seem crazy to me. OpenAI could well be a legit $1T+ company (based on revenue + growth) in the not too distant future.