But more interesting is that world trade intensity didn't back up to the gilded age levels again until around 1975.
...
Quite a bit of the world industrial capacity was destroyed in the 1940s, maybe it took some regions 30 years to get back to full industrial strength?

I'm pretty sure that's not really it. It isn't a measure of global trade, it's a measure of global trade as a fraction of the size of global production. Both trade and GGP shrank, but GGP recovered faster than trade did.

I think it's mainly that trade was simply out of fashion, partly because of perceived risks, despite those risks in absolute terms being vastly below what was considered entirely acceptable in the late 19th century. A part of it may have been the introduction regulation...it was surprisingly easy to invest outside one's own country before the first war (if you were one of the few with the money) because little was prohibited. Later, national brokerages and regulators made it a lot more difficult. It's still not that easy, in general, except for the really big money.

When my (tiny) company was selling stuff in a few Asian countries in 1982, including "red" China as it was then known, I heard we were profitable in more countries than any other Canadian firm. Still, Telex was probably easier than letters sent to an agent by flying boat or clipper ship.

Jim