Axulus, after reading through much of this thread, I think I understand where the confusion is.
When an economist talks about the stagnating middle class, they are not talking about their standards of living. It might be true that the middle class today has bigger houses, more gadgets, better cars, better education, etc. I concede that. But that is not what we are talking about when we talk about the stagnating middle class.
Wealth=assets-debts. That's the key equation. It is the wealth of the middle class that is stagnating. You are equating wealth with assets. Although that might tell us about how the middle class is living, that doesn't tell us how the middle class affects the economy, which is what the economist worries about.
Traditionally, when the middle class has more wealth, they stimulate the economy. They buy more, trade more, and invest more. Every transaction stimulates more transaction.
The middle class is still buying lots of stuff, so it looks like the economy should be stimulated. But this new purchasing is done on credit, i.e. debt. As debt accumulates, the risk of bankruptcy, foreclosure and default heightens. This ultimately has a negative feedback on the market. This is what economists worry about.
As far as the market is concerned, your house, your car, and your livelihood are weighed against your debts. You might enjoy your assets. But the market ultimately doesn't care if you enjoy them. They are just numbers on a sheet of paper. No more or less abstract than your debts.
Standard of living is not wealth.