You should ask people who were planning to retire in 1987 (or 2000, 2001, 2008, or 2020) how they feel about 401ks. Or market investing in general.

It's great that it worked out for you, but there are plenty of people who have taken great losses in the stock market and had to delay retirement. After the dot-com bubble crash in 2000 it took SEVEN years to recover. That also means SEVEN years of no new gains, simply trying to recover what was lost.

The fact is, it is simply one more investing mechanism. And there isn't a financial planner alive who would ever advise you NOT to diversify.

The 401k might be a decent tool for retirement, but it is certainly not the be all and end all.