You're confusing interest with longevity insurance.
I'll repeat, for the umpteenth time -- and it is quite simple, really: interest is paid on all CPF dollars. That includes OA, MA, SA, RA, and the CPF Lifelong Income Fund. As long as you're alive, and for whole calendar months, all interest is paid per CPF's published interest rate schedule.
What your CPF Retirement Account buys is longevity insurance, a life annuity, called CPF LIFE. Whatever portion is allocated to the CPF Lifelong Income Fund still earns interest, and that interest is fully baked into your monthly payment for life. Your life annuity payout claim on the CPF Lifelong Income fund is fully interest inclusive.
OK, so that's what your RA principal and interest buys: a longevity insurance policy, a superb one -- the biggest monthly payout per premium dollar available in Singapore. Now, your longevity insurance policy (CPF LIFE) will then vary in how many months of retirement income it pays to you and how much residual is left (if any). That'll depend on when you start payouts and when you die.
You've repeatedly expressed that you're concerned about your heirs. That's great! It's wonderful you care about them so much. So let me offer some really important advice for how you can help your loved ones:
(a) Insofar as possible, avoid being a financial burden on them.
(b) Insofar as possible, financially help them as early as you can. Don't make them wait until you're gone, when investments in them would be ill-timed or even useless. If somebody needs help to pay a university tuition bill in 2022, don't make them wait until 2032 when they're 10 years older to pay for university. That'd be awful.
Do you know how you can solve both (a) and (b)? Buy at least enough longevity insurance, including especially the policy that provides the highest monthly payout per premium dollars: CPF LIFE. (You're required to buy a little if you can afford it and if you maintain a right of abode in Singapore.) With longevity insurance you are guaranteed you won't outlive your savings. You won't ever be a financial burden to your heirs as long as you can survive on your life annuity income, and you can give away up to every other dollar of wealth you have right now, to whomever you wish, without fear of falling below a certain standard of living. If you love your heirs, and I assume you do, then that's the very best way you can be more generous sooner.