I think we are talking at cross-purposes, but I read your comment as suggesting a single account. No one calls our IRA the CPF MS, CPF always refers to the IRA and Medisave is totally separate even if it is partly administered by the CPF board.

What is the purpose of that tax? In public finance, we don't want to unnecessarily distort incentives by picking winners and losers. Taxing natural resource extraction for no good reason chases away companies in those industries and forces them to pursue other businesses, possibly leaving only the most profitable monopolies to survive. If we want to raise revenue, why not just raise VAT, corporate tax, income tax, or property tax more generally.