Who exactly is projecting 6% returns? Vanguard is projecting 10 year returns of 5.2% U.S. returns before inflation and 8.1% international stocks before inflation. At 2% inflation that's 4.56% real returns if you assume 2% inflation and went 50/ 50 on U.S. and international stock. (Non inflation adjusted returns tell you nothing about readiness for retirement so should be avoided).
And that's at 100% stock, which isn't recommended before retirement.