The team would have more support and a better audience with a larger population. The New York Yankees would gain a larger viewing audience and get more revenues with a larger population. Therefore there is no reason that the New York Yankees should move from New York City if the population exceeds a predetermined limit.
The New York Yankees are a team with a storied history that is deeply entrenched with the city. The idea of moving them - for any reason whatsoever - is simply not a good one. The population of the city should have nothing to do with whether or not the Yankees stay in New York.
They shouldn't have to move just because the population is out of control. The stadium they play at is iconic and it would be silly to make them move cities just because the city is overpopulated. It just is not fair to them and does not make much sense at all for them to move.
No, they do not need to leave New York City, since that is where they have always been and that is where the most of their fans are. It would be a bad idea to go away to anywhere else and risk losing the bulk of your fan base you have.
I do not believe the New York Yankees should move from New York City if the population exceeds a certain predetermined limit. I don't understand why this would be a suggestion at all. To me it seems unimportant what the population of New York would have anything to do with the Yankees.