I agree that this dream of huge returns is luring investors.
I don't think that it will actually work that way. The barriers to making a useful model appear to be modest and keep getting lower. There are a lot of tasks where some AI is useful, but you don't need the very best model if there's a "good enough" solution available at lower prices.
I believe that the irrational exuberance of AI investors is effectively subsidizing technological R&D in this area before AI company valuations drop to realistic levels. Even if OpenAI ends up being analogous to Yahoo! (a currently non-sexy company that was once a darling of investors), their former researchers and engineers can circulate whatever they learned on the job to the organizations that they join later.
This is all embedded in their future growth prospects. Nobody is interested in subsidizing AI as a public service forever. They're interested in "AI is going to make this company go 100x".