Unlike previous years, an increasing proportion of Singapore’s population is foreign-born. Fewer than 2 out of 3 people in this country is a Singapore citizen. Thus, citizens make up a smaller percentage of the total population as each year passes.
Also since this does not take into account the growing percentage of first-generation citizens who are not required to do for NS, the burden falls on an even smaller percentage of the population.
As a result, NS is an institution insists on less than a quarter of the population to bear the burden of defending the rest of the island. This is clearly an unfair notion to many people, especially since there are relatively few substantial benefits associated exclusively with having served NS.
We can think of NS as a kind of tax you have pay upfront as a Singapore-born male citizen so that you can be entitled the minimal benefits (military security and lower taxes) for the rest of your life which you may not even use or is not even guaranteed.
For instance, you may spend a good part of your life working overseas or if the population of eligible male citizens is not sufficient to produce a credible military force twenty years down the road, the government may very well have to raise taxes to hire mercenaries or buy even more expensive weapon systems.
This is a real danger given our falling birth rate. Furthermore, no one can predict the geopolitical situation in SE Asia for the next twenty years.
The current government uses a weak line of reasoning to argue against social welfare benefits for the elderly: our young tax payers should be not paying for welfare benefits for the elderly poor and they should not expect the same when they are old themselves.
If you have assimilated that logic and are able to draw the analogy between tax-funded welfare and NS-supported military defence, then it becomes more difficult to justify NS.