The question is not whether can we defend Singapore; its about whether Singapore is defensible enough for us to defend.
In short, if people do not care so much about the country and ain't passionate about defending it, no matter what we have bought, we would lose. After all the British had far superior numbers on the ground, fairly well-trained troops and had good food supply still. But they still lost once Japan crossed the straits- simply because morale was the main defeating point.
Money is not a substitute for patriotism; but if ordinary citizens do not get the respect that they deserve, and do not get appreciation for what they have done- in terms of praise, cash etc- then, there will be almost no patriotism. Instead everyone would be looking out for their own self-interests instead.
And that's the best gift an invading country will get.