- If your whole app is a wrapper around the GPT-3 API then yes, you don't have much of a moat. But GPT-3 isn't some kind of perfect oracle, so you have lots of engineering challenges in the not-GPT-3 parts of your system where you can build your moat.
- GPT-4 does not render your current efforts moot. It takes time to build business/technical expertise in a domain. That's like saying it was pointless to build a web company in the 90s because web technologies/computing power was increasing so fast. You work with the technology you have, and then when it's time you upgrade.
- I disagree with the idea that you'll have hundreds of competitors. We're just starting to figure out what we can do with this technology. The space is wide open for someone to figure out a novel application.
AI dungeon was mentioned in another comment and serves as a good counter example. Where are all the AI dungeon competitors? How easy would it be to build a competing product to AI dungeon if you wanted to? And AI dungeon started with GPT-2, but incorporated GPT-3 into their product when it came out.
To summarize, starting a business around GPT-3 is a great idea. Stay realistic, be cognizant of GPT-3's limitations, but do realize that there is a genuine opportunity here.