[alanbernstein]

The whole discussion seems to center on the TFP index. I'm not familiar with this, but Wikipedia says "it's also called multi-factor productivity", and it's a ratio of GDP to the "weighted geometric average of labour and capital input". Asking "is growth linear?" is a very different question than "is output/input linear?". Of course it's reasonable for input to grow exponentially as well as output, so yes, the ratio should be linear, which is not inconsistent with just the output being exponential.

And, I can't say without more reading more, what might be hidden by that weighting? Zooming into an exponential enough will make it look linear.