In my view retirement finance is the nastiest problem in finance as you need to deal with inflation, longevity risk, investment returns and spending changes from retirement till the end of life when medical expenses will dominate.
Say from 55 to 95, straight line amortization means P/40 or 40 years of expenses if that amount meets expenses. 2.5% withdrawal rate. Should be good enough to deal with the 4 factors above if P is at least 50% invested in equities.
I think if you're looking at past 70, it can be quite straightforward. Because CPFlife takes away a lot of uncertainty. Especially if you choose Escalating Plan. (Lessens the issue of longevity risk as well as inflation risk.)
If you plan for medical emergencies and perhaps end-of-life care, that will also sort out spending changes (especially since studies show that day-to-day spending probably decreases in the longer run for retirees.)
And if your spending is covered, then investment returns really don't matter. And for those with kids, we can just gift more to them earlier and leave that as a problem for them to solve.
But CPFlife really helps to remove (or lessen) quite a bit of uncertainty for retirement planning.
Though there is probably a cost to having that uncertainty removed. IE. Some may feel that they can do better than CPF life. But that choice will come with more uncertainty.