Buy SPY by Aug 21, 2027.

Let's assume I'm a long-term buy and hold investor invested in index funds. Why is it a bad idea to hold 10-30% of my portfolio in something like SSO to juice up my returns? Over the long term, the S&P 500 will go up (the continued existence of every pension fund in the world depends on this assumption holding true). And if SSO is a bad idea, what's an alternative that can give you some leverage without too much hassle?