I somewhat agree with your point, but also I find that the major agent harnesses work really hard at tricking you into leaning back and not letting a proper understanding of what you are building to develop.

This isn't a new problem. I had it when working with outsource developers, too. In both cases, it seemed that just sitting back and describing what I wanted in plain English prevented me from properly wrapping my head around the problem. Which then meant I failed to develop the insight needed to find the really good improvements. Those usually consisted of identifying and removing the bits that were making things worse.[1] It's incredibly difficult for me to think of anything other than just adding more stuff on top of what's already there if I'm not getting my own hands dirty.

So I've been moving away from plan mode and letting the spin away on building large chunks, and back toward working in small increments and typing out the important code with my own hands. That seems to be the sweet spot. I'm still getting a productivity boost because there's more than enough boilerplate and trivial function definitions to farm out. (Easy prompt, too: "Implement `someFunctionStub`.") But I'm also not letting Plan Mode drag me back into the waterfall development pit of insanity.

1: https://en.wikipedia.org/wiki/No_Silver_Bullet