My brother is a lawyer and told me that eminent domain in NY is fairly easy relative to the rest of the country. Easy as in easy for the government to successfully take the land. We were at a Mets game, parked out near the auto repair shops when there was talk about the city taking the land through eminent domain. He had just finished studying the concept in one of his classes.
However, my guess is MSG has A LOT more resources to fight this if it ever came to it, then the auto repair shop owners. If the city REALLY tried to take the arena, it would be tied up in court for years, so they would likely work something out. Such as giving them VERY favorable terms for the post office.