The biggest issue is government change the SRS policy so that one can withdraw later.
I don't think that's much of a consideration. The government is free to change any tax rules it wishes, including the tax treatment of non-SRS assets.
However, the Supplementary Retirement Scheme only provides potential/likely
Singapore tax advantages. There might be a few other countries that honor Singapore's tax treatment of SRS funds, but most countries don't/won't. If you move to another country, all bets are really off.
I happen to be a U.S. citizen, thus subject to the U.S. tax system no matter where I live, and (by my calculations anyway) Singapore's Supplementary Retirement Scheme doesn't look like it'll work well for me. I might be able to eke out a bit of net tax savings when all is said and done, but the universe of SRS compatible and U.S. tax appropriate investments is a very, very small universe. That's something of a showstopper, so I'm skipping the SRS for now. At the very tail end of my working career I'll take another look since it might make sense then to "launder" some employment income across a very few years pre- and post-retirement, even with the U.S. tax overhang. We'll see.