Ryan Avent at the Economist explains much more clearly than I the wrongness of the claim that low population growth will make us better off:

The point concerning government spending is simply bizarre. Projected growth in federal spending is largely due to rising spending on entitlements, especially Medicare and Medicaid. Slower population growth isn’t going to limit this spending growth; it will just increase the dependency ratio and the expected per capita burden of taxation….

…Indeed, all of the above is precisely what has been observed in Japan, where population growth slowed, halted, and eventually reversed. Per capita incomes have risen only very slowly, government debt is enormous, households are heavy savers, and deflation is endemic.

Also in the comments Andy Harless tries to provide an explanation for what Johnson may be thinking:

(1) fewer immigrants mean less competition for jobs in the short run (assuming immigrants don’t create enough domestic demand to support their employment), and (2) fewer children mean less drain on governments. Of course fewer children do also mean less demand and therefore fewer jobs, but this is obviously endogenous: children are just a manifestation of the multiplier effect, an expense that people choose based on their income. OTOH immigration is also endogenous, so all the first argument is really saying is that it’s a good thing people aren’t dumb enough to keep coming to the US when there are no jobs.

I think this is quite possibly what Johnson is thinking, and I like Andy I think there are some big problems with it. I won’t rehash the arguments here, but there are a lot of other reasons why more immigration would make us better off.

Commenter Adam and Matt Yglesias (via twitter) also point out that an economy based mainly on some scarce natural resource or agriculture could have diminishing returns to labor even in the long-run as capital adjusts, which would explain higher real wages as population grows. However as Matt, Adam, and I agree, this a very bad model of the U.S. economy.

Karl addresses the monetary impacts in the comments:

More worker would imply an increasing demand for money. If you are thinking of the money stock as fixed this will tend to be deflationary and worsen our condition.

However, we have a rate target so increasing money demand should be met by increasing lending. More population should supply more credit unconstrained borrowers who can profitably take out loans at the prevailing interest rate.

Overall I am less puzzled by what Johnson could be thinking, but don’t see a plausible case for why he could be right.