If no NS, who to fight and defend the country from external aggression? All FS/PS holding red passports not likely to hang around to fight and defend SG, likely to throw the red passport into the drain and balek kampong if SG under attack. Only locals born and bred in the country got no choice, got to fight to keep the country safe.
NS is vital to survival of the country and its own people. Foreigners not going to fight for a country that does not belong to them. Even holding red passport, SG is not their homeland. The pappies sadly do not understand why Tiongs when on homeward bound still sings the national anthem of PRC, while in SG they cannot sing or refuse to sing Majullah Singapura.