>>50 years ago, when most US companies were US companies, perhaps only US GDP. But today, where some of (most of?) the largest companies are multinational,

The change is much smaller than most people suppose. Large US companies have always been exporters, even 50+ years ago. IBM, anyone? For example 27% in 1992 vs 29% in 2019 is not exactly a night and day difference.

Meanwhile, there is an offsetting change...revenues of non-US firms firms inside the US.<<

More basically Warren made the comment about "playing with fire" at 140% of GDP when US export numbers were quite high--I believe the comment was made 16-17 years ago. Probably within 1% of today's numbers. And with the tariffs promised-- exports could easily wind up LOWER than when he made the statement. And we're at 200%.