That OpenAI will kill every startup that build a product on top of their technology isn't really a good signal for using OpenAI as a technological platform …
Lots of companies are just wrappers around an API. Can be compared to Apple "killing" startups that sold flashlight apps to the first Iphone
Maybe it's better to build specific applications with OpenAI's products (e.g., using the API to automate a specific business process in your company) than to build a company offering a generic additional functionality like a UI or fine-tuning or custom prompts. Or at least, that seems like the target audience that OpenAI wants to sell their product to.