Stocks are, generally, fairly speculative. If they weren't, everyone would invest in stocks. The person this article imagined had an unfair advantage- he knew that the decades upcoming would see stock upturns, so stocks were not as speculative. Still, stocks just need to beat the rate of inflation. I can easily see them doing that after they've fallen so much.
Well, it's betting on some other factors as well, like the portion of the growth that publicly held firms will capture (versus more of the growth going to new startups or privately held firms). A stock portfolio is by necessity non-diversified on that measure, though large investors can diversify by putting money into VC, angel, and private-equity funds, among other things. It's probably also geographically non-diversified, since some of the fastest-growing markets are hard to buy into (e.g. China is very picky about what kinds of shares it lets foreign investors buy).
I think you'd be surprised at the number of people who don't own stocks and don't even have a 401k or any other type of retirement plan, and I bet they outnumber those who do.
Not really. The resource investment should be the same in both instances, in fact the rich would have an even easier time raising a kid to maturity which is when a child becomes a resource to their parents so I still don't buy it.