And I would say that there are no "phases" of the type you describe. Certain products may go through phases, where they are invented, prototyped, produced, outsourced, and finally obsoleted . . . but the economy is made up of so many different products, each in a different phase, that this all smooths out when you're talking about the big picture.
I think we are getting a bit off-topic here considering the phases I am talking about are big-picture, almost historical changes (are you refuting growth phases like hunting and gathering based economies to agricultural centered economies to manufacturing/industry based economies? To say everything on a macro level was always the same does not make sense. Time periods like the industrial revolution, etc.?
That aside, even if each product goes through phases (which generally I agree with), adding up all those micro phases can result in a macro conclusion. In terms of growth we see this with business cycles- recessions, depressions, growth, bubbles, etc. In terms of tech we see this with the general progression of the market and how much marketshare different sectors occupy.
If it did not, we should be seeing wild fluctuations in employment rates throughout history, and this is not the case.
Well there are business cycles and we do see cyclical changes in employment over time. Either way, if you agree that products can phase out, then clearly new products need to come out at the same time if you think unemployment is always somewhat constant in this regard (which it is not). This means you need innovation- new products don't magically come out- and that is what Cowen argues we might have a shortage of here.
It doesn't matter what the going rate is. If people cannot pay off the debt they already have, they will never sign up for more debt, even if it's loaned out practically free.
People will take on debt if they are given it in the right circumstances

But forget that, I think I am misunderstanding what you are trying to say. Are you claiming that private debt is the problem (I had assumed you were talking about public debt)? Why do you think it is a problem and how is it causing our current unemployment?
I will agree that I believe private saving needs to go up and it naturally is in this downturn, but I think that does not really account for the unemployment.