Claude built by Claude, you mean. It should be trivial to reproduce if you believe the hype.

In the hypothetical world where LLMs have been commoditized and the open weight models are competitive with Claude models, the interface does become the key part!

Despite being awful in so many ways, Microsoft Office continues to dominate. It's a "competitive edge" because is has the trust of corporate customers. There are many competitors, but everybody is used to the main one already, and it's "good enough".