I think there are a few opportunities for startups that want to leverage GPT technology:

1) Fine tuning base models with data that big tech doesn't have access to. E.g legal, medical, support data. Offering custom fine-tuned private hosted models for companies that can't leverage the base models APIs due to data privacy and lack of domain specific training.

2) Using GPT on the backend to do data transformation that the user doesn't interact with directly, e.g parsing logs, events, moderating content etc.

I don't think the opportunity lies in creating a thin wrapper to a custom prompt in a chat interface.