This argument is not unique to CPF. You are free to direct your investment dollars (beyond compulsory CPF contributions) however you wish. With few exceptions you will have no guarantee of any particular yield or even of preservation of capital.
One exception is if you buy Singapore Government Securities and/or Singapore Savings Bonds. However, those bonds and bills are only guaranteed (by the AAA-rated Singapore government) to provide a particular nominal yield and return of principle.(*) If Singapore dollar inflation increases during your holding period then that's just too bad. CPF, on the other hand, will automatically increase its yields based on current benchmark market interest rates. Yes, with the caveat that policymakers could change the interest calculation rules, but see above about no guarantees.
Another possible option is to buy U.S. Treasury Inflation Protected Securities ("TIPS"). TIPS provide a U.S. government guaranteed real return in U.S. dollar terms, based on U.S. dollar inflation. A few other governments with high credit ratings issue bonds similar to TIPS, but TIPS are likely the most accessible such bonds for Singaporean investors.
Of course, you get what you pay for. TIPS, SGSs, and SSBs are all low yielding. Safety (nominal or real) has a price. Also, bear in mind that CPF is unique in offering substantial, immediate Singapore tax relief. That's upfront, thus realized and guaranteed (and also on the backend, under current tax law). Factor that into your calculus, always.
Since you cannot avoid all risks, my advice would be to diversify your portfolio, to a reasonable degree. CPF risks are low in any rational assessment, but of course they are not zero. Nothing offers zero risk. So hold some CPF -- the government already caps voluntary contributions anyway, so nobody well-to-do can overinvest in CPF -- and hold some other stuff, too.
(*) Actually, nominal bond value -- "face value." Also, SGSs are often callable.