You are confusing two different types of analysis. If you had taken a (more rigorous) macroeconomics course you would have learned that there are two types of analysis: long-run and short-run. In the short-run prices are inflexible due to unions, preexisting contracts, menu costs, etc. In the long-run prices can fluctuate and recessions are impossible (we don't worry about some recession that happened in 120 A.D. for example). Keynesian economics only deals with the short run, and only in the short run can the economy be trapped in a temporary disequilibrium. Even though the economy should eventually recover on its own in the long run it can cause a lot of pain in the process, which is why Keynesian economics advocates government intervention.
"I've long had a hobby interest in economics... The inmates are running the asylum."
Maybe a hobbyist shouldn't be suggesting that Nobel prize winners be put in an asylum ;).