So, if the goal is to have residual income when we retire, which seems based on Social Security rules to only be possible in our 60’s, and the government has mandated penalties before taking our money before 59.5, wouldn’t it be prudent to start investing in sources of residual income now that maybe don’t have an age limit into our 60’s? What guarantee do we have that we will make it that long?
One of the things I'm surprised your article doesn't mention is the tax advantages of this type of investment. The depreciation and rehab costs (purchasing distressed properties) can be huge deductions to ones income taxes, which none of the others have. Then, along with the appreciation of real estate, this passive income investment outperforms the notion of maxing out my 401k as well.
What's crazy is that my book income is more than my SF condo-rental income. Yet I didn't have to come up with $1.2 million of capital (the minimum cost to buy my condo today) to create my book. All I needed to create my book was energy, effort, and creativity. I truly believe that developing your own online product is one of the best ways to make money.

If you're looking to get started in real estate, look at a crowd funding solution like RealtyMogul. It works similar to LendingClub - you commit as little as $5,000 towards a property. When the property is fully funded, you become an owner, and will receive your share of the earnings and appreciation in the property.  Check out RealtyMogul to learn more.