GIC prefers to report annualized rolling 20 year real (above inflation) returns. In its mid-2023 report GIC said that figure was 4.6% in U.S. dollar terms. That's way above the 4.08% (current nominal interest rate, excluding bonus interest, in Singapore dollar terms) that the CPF Board pays on Special, MediSave, and Retirement Accounts.
However, you could argue that if the total interest the special CPF bonds pay is higher then there's that much less money that can be reinvested/reserved or disbursed for other public purposes. The latter is called the Net Investment Returns Contribution (NIRC), the portion of Singapore's government budget that the MAS and sovereign wealth funds (GIC, Temasek) support.
I don't think you have to worry about any of this. It's reasonable to assume that the government has powerful incentives to help elder Singaporeans defend against longevity risks, maintain medical savings accounts to help pay for growing medical care needs but with cost control in mind, and probably also save for purchased housing, particularly HDB flats. There are important policy goals in all of that, and CPF interest rates help support those policy goals. Consequently it's reasonable to forecast ongoing, substantial, enduring GIC support for these goals.