There is some risk, but something like SRS cannot be easily discontinued or changed radically - people made the decision to contribute based on the current value proposition.
I could see SRS being enhanced in the future, because it is the 50% taxable rule that causes an imbalance for those early in their career - they are the ones who can more reasonably expect their investments to triple or quadruple over multiple decades. With the 50% tax rule, if your investments grow to more the double what you contributed, each additional dollar is now going to be subject to tax (that would not have been taxed outside SRS). That is the real and present deterrent for not wanting to invest in SRS for too long.
It depends on the purpose of SRS. I don't think it is meant for young workers to save on tax. More an instrument to encourage people to save more for their retirement. Which is first served by CPF anyway.
(Making it too enticing ends up taxing the government instead due to lower taxes being paid. So a balance needs to be struck.)
So in a way, I guess it is targeting those who have maxed out FRS, but still feel that is insufficient for retirement. In other words, these folks have high monthly expenditure over and above what CPF Life would pay them, and likely have high monthly income as well.
So if they contribute when tax rate is 11.5% (Not too high. I'm sure many who contribute are higher), then the full sum of $15,300 would have saved them $1,759 in taxes.
Now, because by the time they draw out SRS, it is likely they are not working, and with the 50% discount, even if they draw out $80,000 (>5x of $15,300...) from SRS, they would only be paying $550 in tax.