In fact for the thousand years until 1950 such extrapolation would place an infinite economy in the late 20th Century! The time since 1950 has been strange apparently.

(Forgive me for playing with anthropics, a tool I do not understand) - maybe something happened to all the observers in worlds that didn't see stagnation set in during the '70s. I guess this is similar to the common joke 'explanation' for the bursting of the tech bubble.