Why can't Singapore be a democracy?

Why can't Singapore be a democracy? I was just thinking about the degree to which people have in-group preferences and realised these two are related.

People will vote for their own ethnic groups. Local Chinese will vote for local Chinese candidates and policies that are good for them. Malaysians will go for Malaysians. It is also possible that the political system could be subverted by the neighbours, such as Indonesia and Malaysia.

It is likely that the society will disintegrate into ethnic factions.

But is this absolutely and certainly true? No, but that's what the political class thinks.