3) The stimulus is either a) causing further problems or b) enhancing the existing ones.


You're forgetting: or c) did nothing or d) helped, but not enough.

I think Keynesianism is useful because, while it may not be effective, it doesn't really hurt anything either. It's impossible for a politician to do nothing during a financial crisis, and this option keeps them from doing something that might really cause harm. Also, this way we get a bunch of infrastructure upgrades that we needed to do anyway.