Yeah, definitely no magical thinking here. Nothing is free. Computers cost money and energy. Infrastructure costs money and energy. Even if no human is in the loop(who says this is even desirable?), all of the things you mention require infrastructure, computers, materials. Meaning there's a cost. Also, the idea that "AI law enforcement" is somehow perfect just goes to illustrates GP's point. Sure, if we define "AGI" as something which can do anything perfectly at no cost, then it has infinite value. But that's not a reasonable definition of AGI. And it's exactly the AI analogue of a perpetual motion machine.
If we can build robots with human level intelligence then you could apply that to all of the costs you describe with substantial savings. Even if such a robot was $100k that is still a one time cost (with maintenance but that’s a fraction of the full price) and long-term substantially cheaper than human workers.
So it’s not just the products that get cheaper, it’s the materials that go into the products that get cheaper too. Heck, what if the robots can build other robots? The cost of that would get cheaper too.