> Comparing manufacturing output between time periods doesn't make a lot sense. How many 1980s mainframes equals one iPhone? It's a woefully incomplete question if you goal is to reason about economic value/output.

I believe that was the point of the "share-of-real-GDP" adjustments. Because "prices of manufactured goods have gone up more slowly than those of other goods and services (think health care), Baily and Bosworth reasoned that manufacturing's shrinking share of nominal GDP understates its continuing impact on the economy."

Apparently there is a decent way, at least in principle, to compare mainframes to iPhones; that is, to compare their contributions to improved productivity independent of nominal prices. But it breaks down over longer time scales unless you can plugin real numbers for changes in relative value-add across entire industries, which in this case isn't possible because the trick was used to adjust industries in the aggregate by assuming a constant ratio of value-add.