That's assuming you want to leave the whole 2 million principal in your will to your children. I don't see why - if you want to retire, you can consume the whole principal while you live.
You can't plan to spend your last dollar on your deathbed unless you can predict every expense, the market, and the day you'll die, especially over a retirement that long (potentially 50+ years). Planning to leave something behind is really your only chance, unless you're comfortable with a very high chance of bankruptcy along the way.
Yes it's a rule of thumb not the law of thermodynamics. Run your numbers with your own parameters for details.