> you lay out a huge specification that would fully work through all of the complexity in advance, then build it.

This has never happened and never will. You simply are not omniscient. Even if you're smart enough to figure everything out the requirements will change underneath you.

But I do still think there's a lot of value into coming up with a good plan before jumping in. A lot of software people like to jump in and I see them portray the planning people as trying to figure everything out first. (I wonder if we reinforce the jumping in head first mentality because people figure out you can't plan everything) A good plan helps you prevent changing specs and prepares you for hiccups. It helps by having others but basically all you do is try to think of all the things that could go wrong. Write them down. Triage. If needed, elevate questions to the decision makers. Try a few small scale tests. Then build out. But building out you're always going to find things you didn't see. You can't plan forever because you'll never solve the unknown unknowns until you build, but also good prep makes for smoother processes. It's the reason engineers do the math before they build a bridge. Not because the math is a perfect representation and things won't change (despite common belief, it's not static) but because the plan is cheaper than the build and having a plan allows you to better track changes and helps you determine how off the rails you've gone.

It is also perplexing to me that people think they can just plan everything out and give it to a LLMs. Do you really believe your manager knows everything that needs to be done when they assign jobs to you? Of course not, they couldn't. Half the job is figuring out what the actual requirements are.