It's always been that way. The crash of 1929 followed a boom. Since the second war we've had numerous recessions that followed boom periods. That's because in a fiat money and credit system the economy gets ahead of itself. These errors, bad decisions and over-exuberant expansion, get punished when business slows and sanity returns.
Nothing goes up forever, and no business or economy expands indefinitely.
The fact that Wall Streeters are now suggesting we can engineer a permanent boom is a tell-tale sign the end is near. Look at Japan. They learned about easy money and credit from the U.S. example. Then we flooded them with dollars. They overbuilt production facilities, drove real estate values into the stratosphere, leveraged themselves internationally and rode their stock market to the moon.