I am not Kent Beck; I don't have all the answers.

All AI has shown us is that we should probably write specs before we code. That was true before AI, but LLMs have just shone a light on this, and it will remain true if agentic coding falls out of fashion again.

Never once did I advocate waterfall...

Read the Royce paper, seriously. It's short, he's a much better writer than I am, and if nothing else it's a fascinating looking at the old state of the art.