There's No Such Thing as Idle Cash on the Sidelines
One of the hurdles in thinking properly about the financial markets is to understand the idea of “equilibrium” – that all securities issued must be held; that savings must equal investment; that every share bought by someone must be sold by someone else.
… and that there's no such thing as “idle cash on the sidelines.”
That last one isn't easy to grasp. After all, you can look at your own brokerage account and say – “look right there at that cash balance. There it is, on the sidelines, just waiting for me to put it into the market.”
But if you look more closely, what you really have is an IOU. It might be a very liquid one, like a money market fund that holds T-bills and commercial paper, but it's still an IOU. See, your “cash on the sidelines” isn't sitting there idle, waiting to be put to work. The fact is that it has already been put to work.
And when you go to put your “cash on the sidelines” to work, what really happens is that your money market securities (T-bills, commercial paper, etc) now have to be sold to someone else. And at that moment, the cash on the sidelines that you had suddenly becomes somebody else's cash on the sidelines. And that same amount of cash on the sidelines will continue to exist until the borrowers pay it off.