The American economy is currently doing so well. Most of the investors from across the world could shift their base to New York. The new Governments policies would encourage manufacturing within USA, protection of jobs in the United States. So there is a possibility that New York will reclaim the number one ranking from China for cities with the most billionaires
A change in the government will help New York reclaim the number one ranking for the most billionaires because most of the candidates agree that tax reform is necessary for this country. With tax reform, people will be more willing to work harder to earn more money, which in turn makes more billionaires that will help New York reclaim its title.
A change in United States government and presidency will not help New York City reclaim the number one ranking from China for cities with the most billionaires. The presidency is not tied to the number of millionaires and vice versa. Thus a government change will have no effect on this ranking.
The Presidency, though powerful, does not have the ability to make New York a plural or China a state. Comparing either a single US city, or possibly even a state, with the most populous nation on earth seems an unreasonable measuring stick for any President. Although perhaps if Trump were to pay enough New Yorkers to build the wall along the Mexican and possibly Canadian borders, that added income would suffice.