If I have understood correctly, you're saying that OpenAI should be forecasting greater revenue than this, if they truly think they will have AIs capable of replacing entire industries. But maybe they're just being cautious in their forecasts?
Suppose I have a 3d printing / nanotechnology company, and I think that a year from now I'll have an unlimited supply of infinity boxes capable of making any material artefact. World manufacturing is worth over US$10 trillion. If I thought I could put it all out of business, by charging just 10% of what current manufacturers charge, I could claim expected revenue of $1 trillion.
Such a prediction would certainly be attention-grabbing, but maybe it would be reckless to make it? Maybe my technology won't be ready. Maybe my products will be blocked from most markets. Maybe someone will reverse-engineer and open-source the infinity boxes, and prices will crash to $0. Maybe I don't want the competition or the government to grasp just how big my plans are. Maybe the investors I want wouldn't believe such a scenario. There are a lot of reasons why a company that thinks it might be able to take over the economy or even the world, would nonetheless not put that in its prospectus.
Hypothesis:
OpenAI believe they won’t be able to lock-in market share. That is, they expect competitors will take a large share of the industry.
What I consider to be a more likely hypothesis: they have to tell investors something that sounds sane to investors, so they are giving them an in-distribution prediction based on the performance of past companies. OpenAI might not believe it, but investors will, and that's all that matters in for the purposes of why they are making these projections.
My hypotheses: