Leveraged funds can be an excellent tool for portfolio construction, for example, products like 100% stocks + 100% bonds (so -100% cash; internally borrowed in the ETF), e.g. RSSB.

And just because it's available doesn't mean it should be your only ETF/ETP. Not a recommendation or advice, but something like 50% TQQQ, and 50% risk-off asset (gold, bonds, whatever); rebalanced regularly isn't crazy, and might even have alpha.