Although you are usually correct (congrats!), I am afraid I have to correct you this time. The CPF Act states that the minimum interest to be paid is 2.5%. The govt can of course amend the Act but I doubt they will so that since they have not done so during the last decade of low interest rate.
The often spoken S&P return is an average. If you buy the S&P index at its height, you will have to wait a very long time for that average to kick in.
So I keep some, but not all, money in CPF earning 2.5% (better than FD) and if the S&P does a significant correction, I will shift some of the CPF money to a low cost S&P etf. That ought to earn me the historical 8% return. Barring that, 2.5% is not the end of the world.
Don't see where I'm corrected? You basically just agreed that anything can change except it's unlikely?
As for the S&P average, yes, it's an average, but if one DCA in, that average still works because it's still an average over a long long time. It doesn't matter where your 1st $1000 goes, it will average out over that 20-30 years if you invest consistently till you retired.
On the other hand, if you managed to dump in 1 lump sum at absolute low or near that, you are likely to get way more than the averaged 8%.
I do agree, however, that one should only use CPF to invest after GFC hits. As to me, cpf is a backup warchest.