I'm inclined to believe that economists are way too optimistic about the prospects for continuing economic and technical progress. The long history of human economic progress tends to fits and starts. An occasional burst raises living standards for a bit, is consumed by Dr. Malthus, leaving all but a select few rather worse off than they were before.
The last couple of hundred years doesn't look like that, but there are pretty good reasons to believe that exponential growth is not a permanent condition. The scientific discoveries that led to the technological explosions of the nineteenth and twentieth century haven't yet run their course, but there isn't much sign of anything truly new to come. There is still a fair amount of juice to be squeezed out of manipulating matter at the molecular level, but then what?
The one revolutionary event still to come - the replacement of humans by robots, is well underway, but that's likely to bring human economics to a finish anyway.