Cities do not, but city states do.
Compare Venice with Monaco in that respect. Or Hong Kong, which is waning because of that lost sovereignity but is still very rich. Or Luxembourg. Or Qatar. It's just great to be a small self-contained free port.
And Venice is not the worst place to be.
Now, we actually do not know what will be of Singapore in 50 years. May be many things.
The point is that cities don't accumulate wealth 'just because'. Wealth comes and goes. And this goes all the more for city-states with no hinterland nor strategic depth.