It is really quite simple, if interest rates are so low that you have to pay only 1/3 of your internal rate of return on assets employed in the business, you want to increase debt exposure.
As much as this is absolutely right, the question is whether the current period is similar to the early sixties in being able to generate that kind of IRR. Intuitively, I don't think it is, but I'd be willing to change my mind.