The human desire for interoperability would prevent writing everything from scratch, but to the extent that is reasonably possible, yes. We would already do that now if humans weren't a limiting factor. But coding agents, as I know the term, is about removing humans from the equation of writing code. I did ask if there is another interpretation for "coding agent" being used.

Of course, distinct projects is a human construct that computers don't care about. As coding agents evolve (if they do), it isn't likely that the idea of projects will persist long-term.