<That's a convenient put-down, but it's based on an 800 year history.
All that matters to me is the previous, say, 30 years, and the next 30 years...
If US companies' profit margins stay elevated for the next 30 years, they can all go to zero in the next century or millenium. I don't care...>

Not a "put-down." Just quoting Sir John Templeton:
"The four most expensive words in the English language are, This time it's different."

And maybe you are correct that the businesses of today deserve higher margins than the businesses of yore and those margins will never go down. Anything is possible.

I tend to agree with Howard Marks, who had this to say recently...

If the declining or ultra-low interest rates of the easy money period aren't going to be the rule in the years ahead numerous consequences seem probable:
Economic growth may be slower
Profit margins may erode
Default rates may head higher
Asset appreciation may not be as reliable
Costs of borrowing will not trend down consistently
Investor psychology may not be as uniformly positive; and
Businesses may not find it so easy to obtain financing.

Much love to all, Banksy out!

oaktreecapital.com - Further thoughts on sea