there's lots of reasons to think that a system that works in Singapore won't scale well

A couple of centuries ago, common wisdom was the opposite - Democracy was a nice idea but it could never work on something bigger than a city. From Montesquieu's Spirit of the Laws:

It is natural for a republic to have only a small territory; otherwise it cannot long subsist. In an extensive republic there are men of large fortunes, and consequently of less moderation; there are trusts too considerable to be placed in any single subject; he has interests of his own; he soon begins to think that he may be happy and glorious, by oppressing his fellow-citizens; and that he may raise himself to grandeur on the ruins of his country.

In an extensive republic the public good is sacrificed to a thousand private views; it is subordinate to exceptions, and depends on accidents. In a small one, the interest of the public is more obvious, better understood, and more within the reach of every citizen; abuses have less extent, and of course are less protected.

[...] Excepting particular circumstances, it is difficult for any other than a republican government to subsist longer in a single town. A prince of so petty a state would naturally endeavour to oppress his subjects, because his power would be great, while the means of enjoying it or of causing it to be respected would be inconsiderable. The consequence is, he would trample upon his people. On the other hand, such a prince might be easily crushed by a foreign or even a domestic force; the people might any instant unite and rise up against him. Now as soon as the sovereign of a single town is expelled, the quarrel is over; but if he has many towns, it only begins.

See also here for more context. During the founding of the US, it wasn't expected that it would work, and then when the French revolution turned into a disaster (followed by similar disasters in Latin America), Democracy seemed less and less of a good idea. The tide only turned in Europe when the US failed to collapse, especially with the publication of Tocqueville's Democracy in America.