well i show never has 50/50 failed out to 30 years and a 96% success rate out to 35 years . 50% s&p 500 and 50% intermediate gov't bonds . maybe you left out dividends
according to michael kitces , Whether the retiree started at the beginning of the Great Depression, entering into the stagflationary recession of the late 1960s and 1970s, or the credit crisis and panic of 1907 and the slow economic growth that followed, an initial withdrawal rate below 4.5% was both necessary and sufficient for the portfolio to survive for 30 years.
in fact here is the exact data
suppose you were so unlucky to retire in one of those worst time framess ,what would your 30 year results look like :
1907 stocks returned 7.77% -- bonds 4.250-- rebalanced portfolio 7.02- - inflation 1.64--
1929 stocks 8.19% - - bonds 1.74%-- rebalanced portfolio 6.28-- inflation 1.69--
1937 stocks 10.12 - - bonds 2.13 - rebalanced portfolio -- 7.24 inflation-- 2.82
1966 stocks 10.23 - -bonds 7.85 -- rebalanced portfolio 9.56- - inflation 5.38
but the portfolio outcomes are determined in the first 15 years .
so here are the 15 year results
1907--- stocks minus 1.47%---- bonds minus .39%-- rebalanced minus .70% ---inflation 1.64%
1929---stocks 1.07%---bonds 1.79%---rebalanced 2.29%--inflation 1.69%
1937---stocks -- 3.45%---bonds minus 3.07%-- rebalanced 1.23%--inflation 2.82%
1966-stocks minus .13%--bonds 1.08%--rebalanced .64%-- inflation 5.38%