When you do most of the thinking before you start implementing the whole thing, and if you think that that's enough, then you've missed the unknown unknowns part, which was a big talking point in the mid 2000s, back when the anti-waterfall discourse got going (and for good reason).

But I expect the AI zealots to start (re-)integrating XProgramming (later rebranded as Agile) back into their workflow, somehow.

Thinking before you start implementing the entire project is doomed to fail. Thinking before you implement each features/user story is usually rather important.

A waterfall model with short feedback loops iterating on small tasks is not the worst thing in the world