It would be fine to delay taking SS...IF a person knew how long they would live. However, IMO, a person can do just as well, perhaps better, by taking SS as soon as they become eligible. I have a statement showing how much I paid in over the years, and I passed the break even point after about 8 years, and now we am getting "other peoples" money. If we live as long as our parents did, we will get at least 3, maybe 4 times more than we paid in....heckuva good deal, and one of the reasons why SS may be in trouble in the future, as more and more people live longer.
Insofar as private investments are concerned...a person MUST educate themselves on how the Stock Market works, and monitor their investments or IRA closely, and be able to shift the funds around as the market conditions change. If they do so, they can stretch an IRA out to give a decent return for many years. If a person is lucky enough to find an Honest financial adviser, they can rest a bit easy, but most "advisers" are in it more for their commissions than the clients well being. Personally, I trust my own decisions, and follow the markets closely, and so far, so good.