It'd mean that hundreds of people would all be goofing off silently. I'd expect at least overtime bookings to decrease and they haven't - even with our strong incentives to not book o/t.
Have you actually tried that? Because my bet is that if your "prototype" is a anything that is very VERY traditional, e.g. a CMS, online shop, or anything that has examples online, yes it will be quick, but if it's genuinely new, namely something NOT available out there, maybe because it is relying on the latest stack that is not yet well documented, then I bet it will also fail terribly.
Edit: I personally did, namely using LLMs to make XR demos relying on a now relatively popular framework https://aframe.io and basically it fails most of the time by proposing "traditional" HTML/CSS, missing entirely that it's 3D. Anyway, long story short, didn't work for me so curious to know if the "getting a prototype" (a genuine prototype, not a codebase starting from scratch because IMHO that's different) part is validated or just an idea.
Never seen it used for anything novel so I can't refute you.
And, crickets. In practice I haven't seen any efficiencies despite my teams using AI in their work. I am not seeing delivery coming in under estimates, work costs what it always cost, we're not doing more stuff or better stuff, and my margins are the same. The only difference I can see is that I've had to negotiate a crapton of contractual amendments to allow my teams to use AI in their work.
I still think it's only good for demos and getting a prototype up and running which is like 5% of any project. Most technical work in enterprise isn't going from zero to something, it's maintaining something, or extending a big, old thing. AI stinks at that (today). You startup people with clean slates may have a different perspective.