Realistically, the data you need for retirement planning doesn't intersect much with double entry personal finance data. How much you spend on subway last week

Realistically, the data you need for retirement planning doesn't intersect much with double entry personal finance data. How much you spend on subway last week has nothing do to with your risk of financial ruin at age 90. All you really need from that dataset is your current portfolio. Everything else you need is specific to investing -- variance of investment, portfolio mix, covariance of returns, age of retirement, life expectancy, etc. From that standpoint, it's practically an entirely new product. There's also something to be said about the 4 percent rule, and whether additional forecasting work actually improves outcomes. Heuristics seem do to a decent job.