Short answer : The internal market doesn't grow.The real problem is that productivity is decoupled from income.
Why is that a problem?
True exports are growing , but imports are growing faster .
Where does the differential come from ?
Debt.
Actually private debt is twice as big as public debt. And debt can't grow forever.
I'm not sure I follow there reasoning here, but it sounds like an explanation for how low wages are harmful. I was specifically asking about the notion that income isn't coupled to productivity. Why should it be? Income is a reflection on the value of the product; the end result is what matters. ie producing something more efficiently won't necessarily result in higher income if the value of the product is declining.