First of all, I am extremely curious how this study counts retirees without investment principal. There are plenty of people who retire on social security and other fixed-income benefits alone. Are they counted as not spending principal (because they don't have it) or are they excluded from the study? Since these retirees might not have much of a choice, I'll focus on those who can adjust their spending strategy based on their investments.

Planning to retire from investment income alone is a very safe strategy for those who can achieve it. It is not easy to save up the amount needed. By the 4% rule, investors should have saved 25 times their annual expenses. According to the research by William Bengen in 1994, withdrawing 4% from investments is sustainable over the span of most retirement horizons. So it is a good goal to have so as to avoid financial stress during one's golden years.

When retiring on investment income alone, it is important to address all the risks. Most people are concerned with stock market risk, and might allocate to bonds in order to secure their retirement income. However, bonds are exposed to inflation risk, which can decrease real spending during retirement. Finally, there is longevity risk, but retiring purely from anticipated income should help minimize this risk. An investment advisor can help design a portfolio to balance these risks.

As for whether to spend principal on retirement expenses, that depends on one's risk appetite and desire to leave a legacy. Spending principal is better suited for retirees without heirs or charitable ambitions.

My standard recommendation is to aim to retire on income alone for the sake of safety. Investment principal could then be used for emergency expenses and legacy purposes.