I don't see why allowing 'rich' people to add to their SA is a bad thing. The thing about being rich is that you have optionality.
If someone with money don't top up SA, they can use their spare CPF-OA or cash to buy Amundi Prime USA / MSCI World funds and get more than 4% return p.a. The 'volatility' of such funds doesn't matter since by the starting premise, they are 'rich' and thus have more financial buffer against volatility.
So those that top up to get 4%, are basically handing government their extra money, which GIC manages and makes more than 4% return. The excess returns are then used to pay for govt programmes, HDB subsidies, build hospitals etc. Its not as if government is 'losing' money whenever someone tops up their CPF-SA.