Did this recession happen because we just ran out of stuff worth doing?

I posted this comment to a Paul Krugman blog posting noting that this recession was different than most recessions in that with this one interest rates low going into it.

One that that struck me over and over again during the Greenspan years was that GDP was growing at a lackluster rate (around 3%) despite big deficit spending and very low interest rates. It was like seeing a person get a huge doses of amphetamines but still have barely enough energy to drag themselves through the day. It made me think our economy must be on the brink of collapse if the economic equivalent of crystal meth was required to just make it seem normal.

Keynes once said that the long term interest rate is determined by what kind of future profit borrowers expect from capital investment. If borrowers see lots of opportunities to make a lot of money, they will be willing to pay more to borrow, and vice versa. In other words, look at interest rates for what they say about demand for borrowed funds instead of what they say about the supply of money.

Putting this together, maybe the story of this recession is that all through the Greenspan years people saw the prospects for profit from real capital investment (investing in new businesses that generate profits from selling new goods and services) getting dimmer and dimmer so interest rates had to be pushed lower and lower to get them to continue to borrow and invest, until the point was reached where there were no worthwhile real capital investments left to invest in and the recession began.

This model isn't inconsistent with the bubbles we saw. It makes sense to me that when one runs out of worthwhile real capital investments then one would start to put easy money into speculating on assets and commodities. If there were ways to make a solid 10% profit from buying new capital assets and opening new businesses would there have been any market for subprime mortgage backed securities?