Too much debt (oh, and let's not forget the impending shortage of oil)
Look how the debt ratio collapsed in the aftermath of the Great Depression. The resulting low debt levels enabled the boom of the 50s, 60s, 80s and 90s. But each upswing needed more debt.
Debt ratios have to decline like they did in the 30s and 40s. And that is a huge structural headwind to longer-term growth.
Disclaimer. After nearly 40 years managing money for some of the largest life offices and investment managers in the world, I think I have something to offer. These days I'm retired, and I can't by law give you advice. While I do make mistakes, I try hard to do my analysis thoroughly, and to make sure my data are correct (old habits die hard!) Also, don't ask me why I called it "Volewica". It's too late, now.
BTW, clicking on most charts will produce the original-sized, i.e., bigger version.