Zero risk return is way below 6% right now. I'm not sure where you're going to find anything like that.
Berkshire Hathaway is willing to guarantee you a $205,000 annual income from that sum in an annuity, which is not a bad deal at all, but it's not 'never worry about money again', it's just a really nice salary, and would be substantially depleted in the case of any inflation at all.
Right now 10-year T-Bills are pulling about 1.65, and inflation is having a nice bounce between 1.7 and 3. Let's even it out at 2 - either way, yes, you're losing money on ultra-low-risk investments. All you can hope for is to reduce the wages of inflation on your balance sheet.
BH annuity is not a bad deal, but losing 2% of your income a year will sound tolerable until about year 5... =)
If you retire mid-twenties on $5M, there's no way you'd make it through 70 on that, if you did nothing else active (read: risky) to increase your income.
Let's assume your investments keep up with inflation but no more, which is very easy to do. Then essentially you are just living off of your principle. 5MM goes a long ways. 75k a year (in today's dollars) will last you 67 years. That's more than most people make. I'd take that.
Follow the thread UP, and you'll see it's the core of the conversation - as to whether one can live off of 6MM for the rest of your life. I call "ceasing working and living on the money you made previously" retirement, you can call it what you like.