When businesses recognize the new technological possibilities, capital spending accelerates to take advantage of the new profit opportunities. Businesses can better produce more output with the same labor input. While supply-side effects are clear, a new higher level of productivity growth would also affect the demand side of the economy.
Of course, some of the causes of the declining debt ratios are not the kinds of things you would typically describe as grounds for optimism. In particular, one reason that many households owe less debt today than in recent years is that banks wrote off an unprecedented amount of loans between 2008 and 2011, much of it as a result of foreclosure or bankruptcy.