Not only are there strict rules about withdrawing 401(k) money prematurely, but your 401(k) could be the most valuable piece of your retirement income pie someday; most people do not have any other employer retirement plan. Today, they remain a popular choice for investors, as they offer a flexible, proven way to save for retirement. At the end of Q2 2021, (the most recent data available), 66% of all Americans participating in retirement plans were invested in 401(k) plans, which held an estimated $7.3 trillion in assets, according to the Investment Company Institute.