However, in real life work situations, that 'perfect information' prerequisite will be a big hurdle I think. Design can depend on any number of vague agreements and lots of domain specific knowledge, things a senior software architect has only learnt because they've been at the company for a long time. It will be very hard for a LLM to take all the correct decisions without that knowledge.
Sure, if you write down a summary of each and every meeting you've attended for the past 12 months, as well as attach your entire company confluence, into the prompt, perhaps then the LLM can design the right architecture. But is that realistic?
More likely I think the human will do the initial design and specification documents, with the aforementioned things in mind, and then the LLM can do the rest of the coding.
Not because it would have been technically impossible for the LLM to do the code design, but because it would have been practically impossible to craft the correct prompt that would have given the desired result from a blank sheet.