What are the consequences to a retiree that relied on the 4% Rule but whose investment returns fell short of that goal?

Why do you think retirement advisors develop such rules-of-thumb?

How would a decrease in such a rule-of-thumb, from 4% to 3% for example, have on the size of the nest-egg that a retiree must accumulate prior to retirement?

How do discussions like this tie into national discussions on Social Security and pension reforms?