I'm not the person you're replying to. But actually I've been getting quite concerned about this for some time. Mostly due to the shrinking world demographics on the horizon. Overall growth is slowing and in most developed economies it will be shrinking. Also, increasing wealth inequality tends to lead to lower overall GDP so if that trend continues I expect lower aggregate growth as well.

I think there is indeed a strong possibility that we may see very poor inflation-adjusted growth from an otherwise reasonable and diverse investment portfolio.

Ultimately, the recipe for growth will just not be so simple in a world economy with a dwindling population. Thats a VERY unique situation so a lot of historical wisdom regarding investments I think may not bear fruit like it did in the past.

My approach to mitigate this is two-fold, first I'm trying to be even MORE diversified. I have investments spread out over domestic and international ETF's, real estate, and I work a public sector job with a public pension. In addition, while aggregate growth may become lackluster, certain industries will still do well. Ive run businesses before and I'm looking to start another business in a very well-targeted industry to add an additional potential revenue stream well into the future. And the second prong of my approach is to increase my savings rate much higher than historically safe targets.

I think there is good reason to be concerned about this and it has very little to do with the current market turmoil. (Although there are some indicators of trouble in that too)