Two foot thick stonework with carved detailing around the doorways?

There probably are more expensive materials though. :)

Metre thick gold with embedded diamonds? Well, 2 metre thick would be even less economic… Good sound deadening, though, I imagine. Next silly context-free question?

An economy is characterised by input of human and natural resources and capital; production and consumption; and barter or exchange of products.

A non-existent internal wall would not involve any of these.

So, an open-plan building with no internal walls, is the least economic. QED.

Edit: Keynes famously said that building pyramids would stimulate the economy by keeping everyone earning and spending. A pyramid has no internal walls as such, it's solid masonry all the way through except maybe a few tomb chambers and a high thermal mass. So a building with no internal walls is simultaneously the most and the least economic.

I am watching an ICF build and find it very surprising that the internal walls are ICF too, wastes money and space

Are they doing things themselves? Putting up the forms yourself and getting concrete pumped might be as cheap as getting blockworkers to build a wall? Or maybe they want to have their bedroom cooler than their living room? Have you asked?

Perhaps they want soundproofing maybe a cinema room.

If the internal walls are supporting the mid span of external walls then maybe its easier to structurally bond ICF to ICF than ICF to block??

What a waste! And mess, I can’t see that happening as it would expose what the concrete looked like.

But yes