I ask because that reaction seems to me to be part of a pattern. It tends to be free market types who are most critical of Malthusian-type pessimism, and I get the impression that it is left-liberal economists who are more worried by secular stagnation whilst rightists are more sanguine about it.
There is, though, no necessary logical connection between optimism and pro-capitalism. Joseph Schumpeter was an admirer of capitalism but feared it could not last. David Ricardo advocated free markets but feared that diminishing returns would choke off growth. And there was for a long time a melancholy tendency within conservatism - expressed most eloquently (pdf) perhaps by Michael Oakeshott.
It would be perfectly coherent to be believe that freeish market capitalism is a good thing but that it is imperiled not just by bad policy but by intrinsic developments such as diminishing returns or a loss of entrepreneurship. And yet free market pessimism is a view that seems to have faded.
Here - in no order - are some theories:
1. My premise is wrong. The fact that index-linked gilt yields are negative suggests that gilt investors, many of whom have pro-market views, are pessimistic about future GDP growth.
This, though, deepens the puzzle of why such a view isn't expressed much in the right-wing media.
2. Optimism is justified. Capitalism has survived many dangers, and has lifted millions out of poverty.
The problem with this is that, as Bertrand Russell famously said, the fact that something has happened in the past does not ensure its continuance. Paradoxically, the optimists might be making the same error as Malthus. Just as he looked at centuries of human history and inferred that mankind was doomed to subsistence, so free market optimists might be over-inferring from two centuries of success. It's quite possible that, at some time, the race between diminishing returns and technical progress will be won by the former.
3. The collapse of communism has led to triumphalism on the right.
The problem here is that centrally planned economies are not the only alternative to capitalism - though many on both left and right fail to see this.
4. Wishful thinking. Just as lefties want to believe that capitalism is doomed because it is bad, so the right wants to believe the opposite.
5.Class. Henrik Cronqvist and colleagues have shown that investors from wealthy families tend to own more growth stocks than those from poor ones, even controlling for their own wealth. This suggests that family backgrounds can predispose people to optimism. For example, Matt Ridley, author of the Rational Optimist, went to Eton - as did one of Mason's critics Douglas Murray. Of course, it's not true that all rightist optimists are posh and pessimists not so, but there might be a tendency here. I conform to it, being very risk-averse and from a poor home.
Points 4 and 5 suggest that what's going on here might not be based wholly upon a rational assessment of the facts. This is, of course, true of both right and left.
For that reason, I don't say this to attack rightist optimism. Instead, I do so to lament the disappearance of an important perspective - that of free market pessimism.