Besides, I think that pessimism (what the physicist probably thinks is realism) rests on one key point, that the price of energy will have a non-zero floor. If you remove that, everything is possible, even assuming steady that energy use, and requiring continued economic growth.
I think the avoidance / supposed maintenance of the "energy price floor" point was too weak. The assumption was that it will still be a purchasable resource ("someone could buy all of it" and hold everyone else hostage), but what if energy becomes unlike that? What if energy in the future is unlimited and no one can buy all of it because everyone can access it? And the only constraint Earth imposes is that no one uses too much so we don't boil off the atmosphere or whatever.
I think that's likely. Zero point energy, free energy from the vacuum. Some whistle-blowers say our secret space program already drives its ships with this kind of power.
What's interesting is that, based on these projections about hitting critical energy output, we won't need this ZPE for another 250 - 600 years. So maybe that's why, if it does exist, it is not going to be released until then. Kind of sad that 40 years ago or so, some bureaucrat planner may have robbed the human race of free energy for the next few centuries because of the projections of an economist and a physicist, not these two, but some other pair tasked with considering the economic and physical implications of then soon-to-be back-engineered alien technology.