Supply and demand always come to equilibrium.
When you muck with supply or demand, things still come to equilibrium, but the economy gets a bit less productive for each intrusion.
If you muck too much, the economy gets a lot less productive.
"Mucking" usually takes the form of granting subsidies or allowing monopoly.
Oh, and rule five. Economics is truly heartless unless you do some mucking about to provide a degree of social justice.