And the policy of transferring money from current workers to retirees -- Social Security, Medicare -- simply isn't sustainable if current workers aren't going to be producing and earning substantially more than those they're subsidizing.
As McArdle writes, "Government accounting is explicitly based on the assumption that spending grows, in real terms, every year -- difficult to achieve unless the economy grows at least as much."
Which suggests a question: Is new-normal slow growth inevitable? Even if you accept Cowen's argument that productivity-enhancing innovation occurs sporadically, can't America do better than it has in the past five (or, if you like, dozen) years?
Barack Obama has been trying to stimulate the economy with record-high government spending funded by higher tax rates and Fed Chairman Ben Bernanke's low interest rates.
But as Stanford economist Michael Boskin points out in The Wall Street Journal, "Japan tried that, to little effect, in the 1990s." Slow growth has become the new normal there.
There are alternative policies. One is to cut government spending, or cut it more than you raise taxes. As Boskin points out, the Netherlands in the mid-1990s and Sweden in the mid-2000s "stabilized their budgets without recession (with) $5-$6 of actual spending cuts per dollar of tax hikes."
And he notes that Canada reduced government spending in the mid-1990s and early 2000s by an amount equal to 8 percent of gross domestic product.
Those cuts weren't painless, but they put Canada on a trajectory different from ours. Canadian voters value budget surpluses, and Canada managed to avoid almost all the bad effects of the 2007-09 recession.
Of course policies can't be transported mechanically from one country to another. Circumstances and customs inevitably differ.
But a strong case can be made that our current policies threaten to make slow growth the new normal. And that would be profoundly painful in ways we are only beginning to imagine.
Republicans are being attacked as irresponsible for allowing the relatively small sequester cuts to occur. But maybe that was the responsible thing to do. Maybe it would be responsible to cut spending even more.