We know the robber-baron industrialists were born in a particular decade, but is that the cause of their success? Obviously not. Gladwell isn't making an argument about the particular reasons that they were lucky -- just observing that they were. They had good timing, and that translated into economic success in their time. Everything else is speculation.
Nobody knows what's going to be "lucky" tomorrow. Maybe this time around, nobody will get rich. Maybe this time, we're creating a new economic paradigm where wealth will be more equitably distributed. Maybe this time, 30-somethings will get rich, because they've had a decade head-start on the 20-somethings to save cash. Maybe this time, everyone will go bust. Maybe this time, risk-takers will get squashed by the market, and savers will be rewarded for their conservative moves. We don't know, and we can't know.
Trying to extrapolate useful information from Gladwell's observation is antithetical to his thesis -- once you've controlled for practice and skill, a lot of the "secret" of success boils down to dumb luck. You can't reproduce luck. The best you can do is work hard, practice a lot, and hope that someday, lightning will strike you too.