It'll be a pretty thin retirement. The interest you get on your money market accounts usually just barely covers inflation and taxes. i.e. the value of your money doesn't grow over time, even though the number on the account statement does.
Run these numbers through a spreadsheet: start with $10,000 and compound by 3% for a typical average money-market like yield, and compare with 8% which is a conservative estimate of stock growth plus dividends. What's the difference after 30 years?
3% starts at $10,000 ends up at $72,817
8% starts at $10,000 ends up at $301,879
that's a difference of over 400%
So, you say in the money market at least you know you've got something for sure after 30 years, whereas the stock market goes up and down and you don't know if it'll be up or down when you need the money. True. But what's the bigger risk: you know for sure that whatever you can buy now with $10,000 will cost you $72,817 in 30 years if inflation is 3% -- that's a 100% probability (that your money-market return will track inflation; that's just the way it works). Or would you rather take the risk that your investment might be down by 40% just when you need it? 40% off of $301,879 is still $181,127, or two and a half times what you'd get from total safety.
Sooooooo.....what's really the bigger risk? Knowing for sure that your money won't grow over time, or knowing for sure that it'll be worth more than sitting in a money-market account, but there's maybe a 20% chance (one in five -- the average bear market cycle) that it'll be worth less than it could have been, had the market not gone down.
1.) Money-market: 100% probability of money not being worth any more than what was saved
2.) Stock market: Perhaps about a 50% chance that it will be worth less than what it peaked out at, and a 20% chance that it will be worth quite a bit less. But virtually (barring nuclear winter or something like that) a 100% chance that it will be worth more than option number one.