The "elasticity of substitution" is an emergent property. It has very little to do with "technology" if only because there is not one single technology in the economy. There are lots of different types of machines and lots of ways to use workers and machines to produce things. And "rigid organization" is not quite right either here: Suresh Naidu (2014): Notes from Capital in the 21st Century Panel: "Perhaps a useful analogy is that this is the "Free to Choose" or “Capitalism and Freedom” for our time, from the left. I can’t think of a book that emerged from economics for a mass audience with as much reception since then. And what good news this is for economics! For 50 years Milton Friedman was the public face of partisan economics, and stamped it with a conservative public face that persisted. Maybe now Piketty’s book will give my discipline another public face...
...But let me push back against the book a bit. I think there is a "domesticated" version of the argument that economists and people that love economists will take away. Then there is a less domesticated one, one that is more challenging to economics as it is currently done. I'm curious which one Thomas believes more. I worry that the impact of the book will be blunted because it becomes a “Bastard Piketty-ism” and allows macroeconomics to continue in its modelling conventions, which are particularly ill-suited to questions of inequality.
The domesticated version is a story about technology and the world market making capital and labor more and more substitutable over time, and this is why r does not fall very much as wealth accumulates. It is fundamentally a story about market forces, technology and trade making the demand for capital extremely elastic. We continue to understand r as the marginal contribution of capital to the production of the economy. I think this is story that is told to academic economists, and it is plausible, at least on the surface.
There is another story about this, one that goes back to Keynes. And the idea here is that the rate of return on capital is set much more by institutions, norms and expectations than by supply and demand of the capital market. Keynes writes that "But the most stable, and the least easily shifted, element in our contemporary economy has been hitherto, and may prove to be in future, the minimum rate of interest acceptable to the generality of wealth-owners." Keynes footnotes it with the 19th century saying that “John Bull can stand many things, but he cannot stand 2 percent.”
The book doesn't quite take a stand on whether it is brute market forces and a production function with a high elasticity of substitution or instead relatively rigid organization of firms and financial institutions that lies behind the stability of r.
I think the production approach is less plausible...