u can retire if passive income exceeds the current FRS/ERS (u choose) payout, and u r confident to sustain it thru the years (test) and cash reserves to last u 2-3 years(u choose) to make a U-turn if cannot. The expenditure is only for self.
Just treat CPF payout as "bonus". or inflation padding. At age 55, take excess (OA, SA closed) to fund ur passive income. ur "job" is to maintain this portfolio payout.
E.g. today, 2016 FRS payout is 1.2k, ERS payout, if available is 2.4k. Can u survive on 2.4k today? If can, then u c next year, can u grow ur payout to 1.35/2.7k. so on and so forth. If cannot, then its moot right, the CPF payout is scholar planned number, they can miss it by alot.
I would like to highlight, may need a large sum (at one shot). When you are on mthly payouts, this is the killer. Large unexpected med bill, or whatever appliance replacement, renov, HDB SERS/VERS, buy niche/"package" etc.