Realistically, if you start a business, there are tons of such dependencies. If that's too scary for you, you're probably too risk adverse to be an entrepreneur.
The issue is not having dependencies. The issue is having one specific dependency that is a beta experimental product with no viable alternative, and which will tank your company when they raise prices.
Your company must always have alternatives for all third party services. The only exception is open source software that you could switch to hosting yourself if the SAAS company shuts down.
Shocking that you are able to even get investors with such a crude view on how business works. Starting an ancillary business related to one product that is not even your own IP or core competency is just lightweight consultancy, not a start up.
The product is entirely just OpenAI. IMO they would only be in the acquisition part to acquire the customer base, nothing about the product. But then they couldn't have another company paying to do the same thing. Why acquire one portfolio when you have _n_ companies paying them to offer same thing to others.
Your company must always have alternatives for all third party services. The only exception is open source software that you could switch to hosting yourself if the SAAS company shuts down.