Correlations change. You never know if gold might tank along with assets in the next recession. Or if yields will tank along with assets in the next recession. What if there's just hyper inflation?

No one knows what's going to happen...

You can go with Dalio's claim that as long as you have 10 hedges that are sufficiently un-correlated, your risk is incredibly low. But even that might not hold up under future unknown conditions.